Propositional calculus is a branch of

/ref> and

formal proof database.

Jan Łukasiewicz
Jan Łukasiewicz (; 21 December 1878 – 13 February 1956) was a Polish logician and philosopher who is best known for Polish notation and Łukasiewicz logic His work centred on philosophical logic, mathematical logic and history of logic. H ...

described above, which is an example of a Hilbert-style deductive system for the classical propositional calculus.
The axioms are:
:(A1) $(p\; \backslash to\; (q\; \backslash to\; p))$
:(A2) $((p\; \backslash to\; (q\; \backslash to\; r))\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; (p\; \backslash to\; r)))$
:(A3) $((\backslash neg\; p\; \backslash to\; \backslash neg\; q)\; \backslash to\; (q\; \backslash to\; p))$
And the proof is as follows:
# $A\; \backslash to\; ((B\; \backslash to\; A)\; \backslash to\; A)$ (instance of (A1))
# $(A\; \backslash to\; ((B\; \backslash to\; A)\; \backslash to\; A))\; \backslash to\; ((A\; \backslash to\; (B\; \backslash to\; A))\; \backslash to\; (A\; \backslash to\; A))$ (instance of (A2))
# $(A\; \backslash to\; (B\; \backslash to\; A))\; \backslash to\; (A\; \backslash to\; A)$ (from (1) and (2) by modus ponens
In propositional logic, ''modus ponens'' (; MP), also known as ''modus ponendo ponens'' (Latin for "method of putting by placing") or implication elimination or affirming the antecedent, is a deductive argument form and rule of inference. ...

)
# $A\; \backslash to\; (B\; \backslash to\; A)$ (instance of (A1))
# $A\; \backslash to\; A$ (from (4) and (3) by modus ponens)

well-formed formula
In mathematical logic, propositional logic and predicate logic, a well-formed formula, abbreviated WFF or wff, often simply formula, is a finite sequence of symbols from a given alphabet that is part of a formal language. A formal language can ...

with the following rules:
* satisfies the propositional variable

modus ponens
In propositional logic, ''modus ponens'' (; MP), also known as ''modus ponendo ponens'' (Latin for "method of putting by placing") or implication elimination or affirming the antecedent, is a deductive argument form and rule of inference. ...

:
:$\backslash frac$.

Eprint

Formal Predicate Calculus

contains a systematic formal development along the lines of Alternative calculus *

forall x: an introduction to formal logic

', by P.D. Magnus, covers formal semantics and

Chapter 2 / Propositional Logic

fro

Logic In Action

Propositional sequent calculus prover

on Project Nayuki. (''note'': implication can be input in the form

Propositional Logic - A Generative Grammar

{{Authority control Systems of formal logic Logical calculi Boolean algebra Classical logic Analytic philosophy

logic
Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from premi ...

. It is also called propositional logic, statement logic, sentential calculus, sentential logic, or sometimes zeroth-order logic. It deals with propositions
In logic and linguistics, a proposition is the meaning of a declarative sentence. In philosophy, " meaning" is understood to be a non-linguistic entity which is shared by all sentences with the same meaning. Equivalently, a proposition is the no ...

(which can be true or false) and relations between propositions, including the construction of arguments based on them. Compound propositions are formed by connecting propositions by logical connective
In logic, a logical connective (also called a logical operator, sentential connective, or sentential operator) is a logical constant. They can be used to connect logical formulas. For instance in the syntax of propositional logic, the binary ...

s. Propositions that contain no logical connectives are called atomic propositions.
Unlike first-order logic
First-order logic—also known as predicate logic, quantificational logic, and first-order predicate calculus—is a collection of formal systems used in mathematics, philosophy, linguistics, and computer science. First-order logic uses quantifie ...

, propositional logic does not deal with non-logical objects, predicates about them, or quantifiers. However, all the machinery of propositional logic is included in first-order logic and higher-order logics. In this sense, propositional logic is the foundation of first-order logic and higher-order logic.
Explanation

Logical connectives are found in natural languages. In English for example, some examples are "and" (conjunction
Conjunction may refer to:
* Conjunction (grammar), a part of speech
* Logical conjunction, a mathematical operator
** Conjunction introduction
Conjunction introduction (often abbreviated simply as conjunction and also called and introduction or ...

), "or" (disjunction
In logic, disjunction is a logical connective typically notated as \lor and read aloud as "or". For instance, the English language sentence "it is raining or it is snowing" can be represented in logic using the disjunctive formula R \lor S ...

), "not" (negation
In logic, negation, also called the logical complement, is an operation that takes a proposition P to another proposition "not P", written \neg P, \mathord P or \overline. It is interpreted intuitively as being true when P is false, and fals ...

) and "if" (but only when used to denote material conditional
The material conditional (also known as material implication) is an operation commonly used in logic. When the conditional symbol \rightarrow is interpreted as material implication, a formula P \rightarrow Q is true unless P is true and Q i ...

).
The following is an example of a very simple inference within the scope of propositional logic:
:Premise 1: If it's raining then it's cloudy.
:Premise 2: It's raining.
:Conclusion: It's cloudy.
Both premises and the conclusion are propositions. The premises are taken for granted, and with the application of modus ponens
In propositional logic, ''modus ponens'' (; MP), also known as ''modus ponendo ponens'' (Latin for "method of putting by placing") or implication elimination or affirming the antecedent, is a deductive argument form and rule of inference. ...

(an inference rule), the conclusion follows.
As propositional logic is not concerned with the structure of propositions beyond the point where they can't be decomposed any more by logical connectives, this inference can be restated replacing those ''atomic'' statements with statement letters, which are interpreted as variables representing statements:
:Premise 1: $P\; \backslash to\; Q$
:Premise 2: $P$
:Conclusion: $Q$
The same can be stated succinctly in the following way:
:$\backslash frac$
When is interpreted as "It's raining" and as "it's cloudy" the above symbolic expressions can be seen to correspond exactly with the original expression in natural language. Not only that, but they will also correspond with any other inference of this ''form'', which will be valid on the same basis this inference is.
Propositional logic may be studied through a formal system
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system.
A for ...

in which formulas of a formal language
In logic, mathematics, computer science, and linguistics, a formal language consists of words whose letters are taken from an alphabet and are well-formed according to a specific set of rules.
The alphabet of a formal language consists of symb ...

may be interpreted to represent propositions
In logic and linguistics, a proposition is the meaning of a declarative sentence. In philosophy, " meaning" is understood to be a non-linguistic entity which is shared by all sentences with the same meaning. Equivalently, a proposition is the no ...

. A system
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...

of axiom
An axiom, postulate, or assumption is a statement that is taken to be true, to serve as a premise or starting point for further reasoning and arguments. The word comes from the Ancient Greek word (), meaning 'that which is thought worthy or ...

s and inference rules allows certain formulas to be derived. These derived formulas are called theorem
In mathematics, a theorem is a statement that has been proved, or can be proved. The ''proof'' of a theorem is a logical argument that uses the inference rules of a deductive system to establish that the theorem is a logical consequence of the ...

s and may be interpreted to be true propositions. A constructed sequence of such formulas is known as a ''derivation
Derivation may refer to:
Language
* Morphological derivation, a word-formation process
* Parse tree or concrete syntax tree, representing a string's syntax in formal grammars
Law
* Derivative work, in copyright law
* Derivation proceeding, a p ...

'' or ''proof'' and the last formula of the sequence is the theorem. The derivation may be interpreted as proof of the proposition represented by the theorem.
When a formal system is used to represent formal logic, only statement letters (usually capital roman letters such as $P$, $Q$ and $R$) are represented directly. The natural language propositions that arise when they're interpreted are outside the scope of the system, and the relation between the formal system and its interpretation is likewise outside the formal system itself.
In classical truth-functional propositional logic, formulas are interpreted as having precisely one of two possible truth value
In logic and mathematics, a truth value, sometimes called a logical value, is a value indicating the relation of a proposition to truth, which in classical logic has only two possible values (''true'' or ''false'').
Computing
In some program ...

s, the truth value of ''true'' or the truth value of ''false''. The principle of bivalence
In logic, the semantic principle (or law) of bivalence states that every declarative sentence expressing a proposition (of a theory under inspection) has exactly one truth value, either true or false. A logic satisfying this principle is calle ...

and the law of excluded middle are upheld. Truth-functional propositional logic defined as such and systems isomorphic
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...

to it are considered to be zeroth-order logic. However, alternative propositional logics are also possible. For more, see Other logical calculi below.
History

Although propositional logic (which is interchangeable with propositional calculus) had been hinted by earlier philosophers, it was developed into a formal logic (Stoic logic
Stoic logic is the system of propositional logic developed by the Stoic philosophers in ancient Greece.
It was one of the two great systems of logic in the classical world. It was largely built and shaped by Chrysippus, the third head of the Stoi ...

) by Chrysippus
Chrysippus of Soli (; grc-gre, Χρύσιππος ὁ Σολεύς, ; ) was a Greek Stoic philosopher. He was a native of Soli, Cilicia, but moved to Athens as a young man, where he became a pupil of the Stoic philosopher Cleanthes. When Cle ...

in the 3rd century BC and expanded by his successor Stoics
Stoicism is a school of Hellenistic philosophy founded by Zeno of Citium in Athens in the early 3rd century BCE. It is a philosophy of personal virtue ethics informed by its system of logic and its views on the natural world, asserting that th ...

. The logic was focused on proposition
In logic and linguistics, a proposition is the meaning of a declarative sentence. In philosophy, " meaning" is understood to be a non-linguistic entity which is shared by all sentences with the same meaning. Equivalently, a proposition is the no ...

s. This advancement was different from the traditional syllogistic logic, which was focused on terms. However, most of the original writings were lost and the propositional logic developed by the Stoics was no longer understood later in antiquity. Consequently, the system was essentially reinvented by Peter Abelard
Peter Abelard (; french: link=no, Pierre Abélard; la, Petrus Abaelardus or ''Abailardus''; 21 April 1142) was a medieval French scholastic philosopher, leading logician, theologian, poet, composer and musician. This source has a detailed des ...

in the 12th century.
Propositional logic was eventually refined using symbolic logic. The 17th/18th-century mathematician Gottfried Leibniz
Gottfried Wilhelm (von) Leibniz . ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist and diplomat. He is one of the most prominent figures in both the history of philosophy and the history of mathem ...

has been credited with being the founder of symbolic logic for his work with the calculus ratiocinator
The ''calculus ratiocinator'' is a theoretical universal logical calculation framework, a concept described in the writings of Gottfried Leibniz, usually paired with his more frequently mentioned ''characteristica universalis'', a universal conce ...

. Although his work was the first of its kind, it was unknown to the larger logical community. Consequently, many of the advances achieved by Leibniz were recreated by logicians like George Boole and Augustus De Morgan—completely independent of Leibniz.
Just as propositional logic can be considered an advancement from the earlier syllogistic logic, Gottlob Frege's predicate logic
First-order logic—also known as predicate logic, quantificational logic, and first-order predicate calculus—is a collection of formal systems used in mathematics, philosophy, linguistics, and computer science. First-order logic uses quantifie ...

can be also considered an advancement from the earlier propositional logic. One author describes predicate logic as combining "the distinctive features of syllogistic logic and propositional logic." Consequently, predicate logic ushered in a new era in logic's history; however, advances in propositional logic were still made after Frege, including natural deduction, truth trees and truth tables
A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional argume ...

. Natural deduction was invented by Gerhard Gentzen and Jan Łukasiewicz
Jan Łukasiewicz (; 21 December 1878 – 13 February 1956) was a Polish logician and philosopher who is best known for Polish notation and Łukasiewicz logic His work centred on philosophical logic, mathematical logic and history of logic. H ...

. Truth trees were invented by Evert Willem Beth
Evert Willem Beth (7 July 1908 – 12 April 1964) was a Dutch philosopher and logician, whose work principally concerned the foundations of mathematics. He was a member of the Significs Group.
Biography
Beth was born in Almelo, a small t ...

. The invention of truth tables, however, is of uncertain attribution.
Within works by FregeTruth in Frege/ref> and

Bertrand Russell
Bertrand Arthur William Russell, 3rd Earl Russell, (18 May 1872 – 2 February 1970) was a British mathematician, philosopher, logician, and public intellectual. He had a considerable influence on mathematics, logic, set theory, linguistics, ar ...

, are ideas influential to the invention of truth tables. The actual tabular structure (being formatted as a table), itself, is generally credited to either Ludwig Wittgenstein
Ludwig Josef Johann Wittgenstein ( ; ; 26 April 1889 – 29 April 1951) was an Austrian- British philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language. He is conside ...

or Emil Post
Emil Leon Post (; February 11, 1897 – April 21, 1954) was an American mathematician and logician. He is best known for his work in the field that eventually became known as computability theory.
Life
Post was born in Augustów, Suwałki Go ...

(or both, independently). Besides Frege and Russell, others credited with having ideas preceding truth tables include Philo, Boole, Charles Sanders Peirce
Charles Sanders Peirce ( ; September 10, 1839 – April 19, 1914) was an American philosopher, logician, mathematician and scientist who is sometimes known as "the father of pragmatism".
Educated as a chemist and employed as a scientist for ...

, and Ernst Schröder. Others credited with the tabular structure include Jan Łukasiewicz
Jan Łukasiewicz (; 21 December 1878 – 13 February 1956) was a Polish logician and philosopher who is best known for Polish notation and Łukasiewicz logic His work centred on philosophical logic, mathematical logic and history of logic. H ...

, Alfred North Whitehead
Alfred North Whitehead (15 February 1861 – 30 December 1947) was an English mathematician and philosopher. He is best known as the defining figure of the philosophical school known as process philosophy, which today has found applica ...

, William Stanley Jevons
William Stanley Jevons (; 1 September 183513 August 1882) was an English economist and logician.
Irving Fisher described Jevons's book ''A General Mathematical Theory of Political Economy'' (1862) as the start of the mathematical method in eco ...

, John Venn
John Venn, FRS, FSA (4 August 1834 – 4 April 1923) was an English mathematician, logician and philosopher noted for introducing Venn diagrams, which are used in logic, set theory, probability, statistics, and computer science. In 1866, ...

, and Clarence Irving Lewis. Ultimately, some have concluded, like John Shosky, that "It is far from clear that any one person should be given the title of 'inventor' of truth-tables.".
Terminology

In general terms, a calculus is aformal system
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system.
A for ...

that consists of a set of syntactic expressions (''well-formed formula
In mathematical logic, propositional logic and predicate logic, a well-formed formula, abbreviated WFF or wff, often simply formula, is a finite sequence of symbols from a given alphabet that is part of a formal language. A formal language can ...

s''), a distinguished subset of these expressions (axioms), plus a set of formal rules that define a specific binary relation
In mathematics, a binary relation associates elements of one set, called the ''domain'', with elements of another set, called the ''codomain''. A binary relation over sets and is a new set of ordered pairs consisting of elements in and in ...

, intended to be interpreted as logical equivalence
In logic and mathematics, statements p and q are said to be logically equivalent if they have the same truth value in every model. The logical equivalence of p and q is sometimes expressed as p \equiv q, p :: q, \textsfpq, or p \iff q, depending on ...

, on the space of expressions.
When the formal system is intended to be a logical system
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system.
A for ...

, the expressions are meant to be interpreted as statements, and the rules, known to be ''inference rules'', are typically intended to be truth-preserving. In this setting, the rules, which may include axioms
An axiom, postulate, or assumption is a statement that is taken to be true, to serve as a premise or starting point for further reasoning and arguments. The word comes from the Ancient Greek word (), meaning 'that which is thought worthy or ...

, can then be used to derive ("infer") formulas representing true statements—from given formulas representing true statements.
The set of axioms may be empty, a nonempty finite set, or a countably infinite set (see axiom schema In mathematical logic, an axiom schema (plural: axiom schemata or axiom schemas) generalizes the notion of axiom.
Formal definition
An axiom schema is a formula in the metalanguage of an axiomatic system, in which one or more schematic variables ...

). A formal grammar
In formal language theory, a grammar (when the context is not given, often called a formal grammar for clarity) describes how to form strings from a language's alphabet that are valid according to the language's syntax. A grammar does not describe ...

recursively defines the expressions and well-formed formulas of the language
Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of me ...

. In addition a semantics
Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...

may be given which defines truth and valuations (or interpretations).
The language
Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of me ...

of a propositional calculus consists of
# a set of primitive symbols, variously referred to as ''atomic formula
In mathematical logic, an atomic formula (also known as an atom or a prime formula) is a formula with no deeper propositional structure, that is, a formula that contains no logical connectives or equivalently a formula that has no strict subformu ...

s'', ''placeholders'', ''proposition letters'', or ''variables'', and
# a set of operator symbols, variously interpreted as ''logical operator
In logic, a logical connective (also called a logical operator, sentential connective, or sentential operator) is a logical constant. They can be used to connect logical formulas. For instance in the syntax of propositional logic, the binary c ...

s'' or ''logical connectives''.
A ''well-formed formula
In mathematical logic, propositional logic and predicate logic, a well-formed formula, abbreviated WFF or wff, often simply formula, is a finite sequence of symbols from a given alphabet that is part of a formal language. A formal language can ...

'' is any atomic formula, or any formula that can be built up from atomic formulas by means of operator symbols according to the rules of the grammar.
Mathematicians sometimes distinguish between propositional constants, propositional variables, and schemata. Propositional constants represent some particular proposition, while propositional variables range over the set of all atomic propositions. Schemata, however, range over all propositions. It is common to represent propositional constants by , , and , propositional variables by , , and , and schematic letters are often Greek letters, most often , , and .
Basic concepts

The following outlines a standard propositional calculus. Many different formulations exist which are all more or less equivalent, but differ in the details of: # their language (i.e., the particular collection of primitive symbols and operator symbols), # the set of axioms, or distinguished formulas, and # the set of inference rules. Any given proposition may be represented with a letter called a 'propositional constant', analogous to representing a number by a letter in mathematics (e.g., ). All propositions require exactly one of two truth-values: true or false. For example, let be the proposition that it is raining outside. This will be true () if it is raining outside, and false otherwise (). *We then define truth-functional operators, beginning with negation. represents the negation of , which can be thought of as the denial of . In the example above, expresses that it is not raining outside, or by a more standard reading: "It is not the case that it is raining outside." When is true, is false; and when is false, is true. As a result, always has the same truth-value as . *Conjunction is a truth-functional connective which forms a proposition out of two simpler propositions, for example, and . The conjunction of and is written , and expresses that each are true. We read as " and ". For any two propositions, there are four possible assignments of truth values: *# is true and is true *# is true and is false *# is false and is true *# is false and is false :The conjunction of and is true in case 1, and is false otherwise. Where is the proposition that it is raining outside and is the proposition that a cold-front is over Kansas, is true when it is raining outside ''and'' there is a cold-front over Kansas. If it is not raining outside, then is false; and if there is no cold-front over Kansas, then is also false. *Disjunction resembles conjunction in that it forms a proposition out of two simpler propositions. We write it , and it is read " or ". It expresses that either or is true. Thus, in the cases listed above, the disjunction of with is true in all cases—except case 4. Using the example above, the disjunction expresses that it is either raining outside, or there is a cold front over Kansas. (Note, this use of disjunction is supposed to resemble the use of the English word "or". However, it is most like the English inclusive "or", which can be used to express the truth of at least one of two propositions. It is not like the English exclusive "or", which expresses the truth of exactly one of two propositions. In other words, the exclusive "or" is false when both and are true (case 1), and similarly is false when both and are false (case 4). An example of the exclusive or is: You may have a bagel or a pastry, but not both. Often in natural language, given the appropriate context, the addendum "but not both" is omitted—but implied. In mathematics, however, "or" is always inclusive or; if exclusive or is meant it will be specified, possibly by "xor".) *Material conditional also joins two simpler propositions, and we write , which is read "if then ". The proposition to the left of the arrow is called the antecedent, and the proposition to the right is called the consequent. (There is no such designation for conjunction or disjunction, since they arecommutative
In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Most familiar as the name of ...

operations.) It expresses that is true whenever is true. Thus is true in every case above except case 2, because this is the only case when is true but is not. Using the example, if then expresses that if it is raining outside, then there is a cold-front over Kansas. The material conditional is often confused with physical causation. The material conditional, however, only relates two propositions by their truth-values—which is not the relation of cause and effect. It is contentious in the literature whether the material implication represents logical causation.
*Biconditional joins two simpler propositions, and we write , which is read " if and only if ". It expresses that and have the same truth-value, and in cases 1 and 4. ' is true if and only if ' is true, and is false otherwise.
It is very helpful to look at the truth table
A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional argumen ...

s for these different operators, as well as the method of analytic tableaux.
Closure under operations

Propositional logic is closed under truth-functional connectives. That is to say, for any proposition , is also a proposition. Likewise, for any propositions and , is a proposition, and similarly for disjunction, conditional, and biconditional. This implies that, for instance, is a proposition, and so it can be conjoined with another proposition. In order to represent this, we need to use parentheses to indicate which proposition is conjoined with which. For instance, is not a well-formed formula, because we do not know if we are conjoining with or if we are conjoining with . Thus we must write either to represent the former, or to represent the latter. By evaluating the truth conditions, we see that both expressions have the same truth conditions (will be true in the same cases), and moreover that any proposition formed by arbitrary conjunctions will have the same truth conditions, regardless of the location of the parentheses. This means that conjunction isassociative
In mathematics, the associative property is a property of some binary operations, which means that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement ...

, however, one should not assume that parentheses never serve a purpose. For instance, the sentence does not have the same truth conditions of , so they are different sentences distinguished only by the parentheses. One can verify this by the truth-table method referenced above.
Note: For any arbitrary number of propositional constants, we can form a finite number of cases which list their possible truth-values. A simple way to generate this is by truth-tables, in which one writes , , ..., , for any list of propositional constants—that is to say, any list of propositional constants with entries. Below this list, one writes rows, and below one fills in the first half of the rows with true (or T) and the second half with false (or F). Below one fills in one-quarter of the rows with T, then one-quarter with F, then one-quarter with T and the last quarter with F. The next column alternates between true and false for each eighth of the rows, then sixteenths, and so on, until the last propositional constant varies between T and F for each row. This will give a complete listing of cases or truth-value assignments possible for those propositional constants.
Argument

The propositional calculus then defines an ''argument
An argument is a statement or group of statements called premises intended to determine the degree of truth or acceptability of another statement called conclusion. Arguments can be studied from three main perspectives: the logical, the dialect ...

'' to be a list of propositions. A valid argument is a list of propositions, the last of which follows from—or is implied by—the rest. All other arguments are invalid. The simplest valid argument is modus ponens
In propositional logic, ''modus ponens'' (; MP), also known as ''modus ponendo ponens'' (Latin for "method of putting by placing") or implication elimination or affirming the antecedent, is a deductive argument form and rule of inference. ...

, one instance of which is the following list of propositions:
:$\backslash begin\; 1.\; \&\; P\; \backslash to\; Q\; \backslash \backslash \; 2.\; \&\; P\; \backslash \backslash \; \backslash hline\; \backslash therefore\; \&\; Q\; \backslash end$
This is a list of three propositions, each line is a proposition, and the last follows from the rest. The first two lines are called premises, and the last line the conclusion. We say that any proposition follows from any set of propositions $(P\_1,\; ...,\; P\_n)$, if must be true whenever every member of the set $(P\_1,\; ...,\; P\_n)$ is true. In the argument above, for any and , whenever and are true, necessarily is true. Notice that, when is true, we cannot consider cases 3 and 4 (from the truth table). When is true, we cannot consider case 2. This leaves only case 1, in which is also true. Thus is implied by the premises.
This generalizes schematically. Thus, where and may be any propositions at all,
:$\backslash begin\; 1.\; \&\; \backslash varphi\; \backslash to\; \backslash psi\; \backslash \backslash \; 2.\; \&\; \backslash varphi\; \backslash \backslash \; \backslash hline\; \backslash therefore\; \&\; \backslash psi\; \backslash end$
Other argument forms are convenient, but not necessary. Given a complete set of axioms (see below for one such set), modus ponens is sufficient to prove all other argument forms in propositional logic, thus they may be considered to be a derivative. Note, this is not true of the extension of propositional logic to other logics like first-order logic
First-order logic—also known as predicate logic, quantificational logic, and first-order predicate calculus—is a collection of formal systems used in mathematics, philosophy, linguistics, and computer science. First-order logic uses quantifie ...

. First-order logic requires at least one additional rule of inference in order to obtain completeness.
The significance of argument in formal logic is that one may obtain new truths from established truths. In the first example above, given the two premises, the truth of is not yet known or stated. After the argument is made, is deduced. In this way, we define a deduction system to be a set of all propositions that may be deduced from another set of propositions. For instance, given the set of propositions $A\; =\; \backslash $, we can define a deduction system, , which is the set of all propositions which follow from . Reiteration is always assumed, so $P\; \backslash lor\; Q,\; \backslash neg\; Q\; \backslash land\; R,\; (P\; \backslash lor\; Q)\; \backslash to\; R\; \backslash in\; \backslash Gamma$. Also, from the first element of , last element, as well as modus ponens, is a consequence, and so $R\; \backslash in\; \backslash Gamma$. Because we have not included sufficiently complete axioms, though, nothing else may be deduced. Thus, even though most deduction systems studied in propositional logic are able to deduce $(P\; \backslash lor\; Q)\; \backslash leftrightarrow\; (\backslash neg\; P\; \backslash to\; Q)$, this one is too weak to prove such a proposition.
Generic description of a propositional calculus

A propositional calculus is aformal system
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system.
A for ...

$\backslash mathcal\; =\; \backslash mathcal\; \backslash left(\; \backslash Alpha,\backslash \; \backslash Omega,\backslash \; \backslash Zeta,\backslash \; \backslash Iota\; \backslash right)$, where:
The ''language'' of $\backslash mathcal$, also known as its set of ''formulas'', ''well-formed formula
In mathematical logic, propositional logic and predicate logic, a well-formed formula, abbreviated WFF or wff, often simply formula, is a finite sequence of symbols from a given alphabet that is part of a formal language. A formal language can ...

s'', is inductively defined by the following rules:
# Base: Any element of the alpha set $\backslash Alpha$ is a formula of $\backslash mathcal$.
# If $p\_1,\; p\_2,\; \backslash ldots,\; p\_j$ are formulas and $f$ is in $\backslash Omega\_j$, then $\backslash left(\; f\; p\_1\; p\_2\; \backslash ldots\; p\_j\; \backslash right)$ is a formula.
# Closed: Nothing else is a formula of $\backslash mathcal$.
Repeated applications of these rules permits the construction of complex formulas. For example:
* By rule 1, is a formula.
* By rule 2, $\backslash neg\; p$ is a formula.
* By rule 1, is a formula.
* By rule 2, $(\; \backslash neg\; p\; \backslash lor\; q\; )$ is a formula.
Example 1. Simple axiom system

Let $\backslash mathcal\_1\; =\; \backslash mathcal(\backslash Alpha,\backslash Omega,\backslash Zeta,\backslash Iota)$, where $\backslash Alpha$, $\backslash Omega$, $\backslash Zeta$, $\backslash Iota$ are defined as follows: * The set $\backslash Alpha$, the countably infinite set of symbols that serve to represent logical propositions: *: $\backslash Alpha\; =\; \backslash .$ * The functionally complete set $\backslash Omega$ of logical operators (logical connectives and negation) is as follows. Of the three connectives for conjunction, disjunction, and implication ($\backslash wedge,\; \backslash lor$, and ), one can be taken as primitive and the other two can be defined in terms of it and negation ().Wernick, William (1942) "Complete Sets of Logical Functions," ''Transactions of the American Mathematical Society'' 51, pp. 117–132. Alternatively, all of the logical operators may be defined in terms of a sole sufficient operator, such as the Sheffer stroke (nand). The biconditional ($a\; \backslash leftrightarrow\; b$) can of course be defined in terms of conjunction and implication as $(a\; \backslash to\; b)\; \backslash land\; (b\; \backslash to\; a)$. Adopting negation and implication as the two primitive operations of a propositional calculus is tantamount to having the omega set $\backslash Omega\; =\; \backslash Omega\_1\; \backslash cup\; \backslash Omega\_2$ partition as follows: *: $\backslash Omega\_1\; =\; \backslash ,$ *: $\backslash Omega\_2\; =\; \backslash .$ Then $a\; \backslash lor\; b$ is defined as $\backslash neg\; a\; \backslash to\; b$, and $a\; \backslash land\; b$ is defined as $\backslash neg(a\; \backslash to\; \backslash neg\; b)$. * The set $\backslash Iota$ (the set of initial points of logical deduction, i.e., logical axioms) is the axiom system proposed byJan Łukasiewicz
Jan Łukasiewicz (; 21 December 1878 – 13 February 1956) was a Polish logician and philosopher who is best known for Polish notation and Łukasiewicz logic His work centred on philosophical logic, mathematical logic and history of logic. H ...

, and used as the propositional-calculus part of a Hilbert system
:''In mathematical physics, ''Hilbert system'' is an infrequently used term for a physical system described by a C*-algebra.''
In logic, especially mathematical logic, a Hilbert system, sometimes called Hilbert calculus, Hilbert-style deductive s ...

. The axioms are all substitution instances of:
** $p\; \backslash to\; (q\; \backslash to\; p)$
** $(p\; \backslash to\; (q\; \backslash to\; r))\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; (p\; \backslash to\; r))$
** $(\backslash neg\; p\; \backslash to\; \backslash neg\; q)\; \backslash to\; (q\; \backslash to\; p)$
* The set $\backslash Zeta$ of transformation rules (rules of inference) is the sole rule modus ponens
In propositional logic, ''modus ponens'' (; MP), also known as ''modus ponendo ponens'' (Latin for "method of putting by placing") or implication elimination or affirming the antecedent, is a deductive argument form and rule of inference. ...

(i.e., from any formulas of the form $\backslash varphi$ and $(\backslash varphi\; \backslash to\; \backslash psi)$, infer $\backslash psi$).
This system is used in Metamathbr>set.mmformal proof database.

Example 2. Natural deduction system

Let $\backslash mathcal\_2\; =\; \backslash mathcal(\backslash Alpha,\; \backslash Omega,\; \backslash Zeta,\; \backslash Iota)$, where $\backslash Alpha$, $\backslash Omega$, $\backslash Zeta$, $\backslash Iota$ are defined as follows: * The alpha set $\backslash Alpha$, is a countably infinite set of symbols, for example: *: $\backslash Alpha\; =\; \backslash .$ * The omega set $\backslash Omega\; =\; \backslash Omega\_1\; \backslash cup\; \backslash Omega\_2$ partitions as follows: *: $\backslash Omega\_1\; =\; \backslash ,$ *: $\backslash Omega\_2\; =\; \backslash .$ In the following example of a propositional calculus, the transformation rules are intended to be interpreted as the inference rules of a so-called '' natural deduction system''. The particular system presented here has no initial points, which means that its interpretation for logical applications derives itstheorem
In mathematics, a theorem is a statement that has been proved, or can be proved. The ''proof'' of a theorem is a logical argument that uses the inference rules of a deductive system to establish that the theorem is a logical consequence of the ...

s from an empty axiom set.
* The set of initial points is empty, that is, $\backslash Iota\; =\; \backslash varnothing$.
* The set of transformation rules, $\backslash Zeta$, is described as follows:
Our propositional calculus has eleven inference rules. These rules allow us to derive other true formulas given a set of formulas that are assumed to be true. The first ten simply state that we can infer certain well-formed formulas from other well-formed formulas. The last rule however uses hypothetical reasoning in the sense that in the premise of the rule we temporarily assume an (unproven) hypothesis to be part of the set of inferred formulas to see if we can infer a certain other formula. Since the first ten rules don't do this they are usually described as ''non-hypothetical'' rules, and the last one as a ''hypothetical'' rule.
In describing the transformation rules, we may introduce a metalanguage symbol $\backslash vdash$. It is basically a convenient shorthand for saying "infer that". The format is $\backslash Gamma\; \backslash vdash\; \backslash psi$, in which is a (possibly empty) set of formulas called premises, and is a formula called conclusion. The transformation rule $\backslash Gamma\; \backslash vdash\; \backslash psi$ means that if every proposition in is a theorem (or has the same truth value as the axioms), then is also a theorem. Note that considering the following rule Conjunction introduction
Conjunction introduction (often abbreviated simply as conjunction and also called and introduction or adjunction) is a valid rule of inference of propositional logic. The rule makes it possible to introduce a conjunction into a logical proof ...

, we will know whenever has more than one formula, we can always safely reduce it into one formula using conjunction. So for short, from that time on we may represent as one formula instead of a set. Another omission for convenience is when is an empty set, in which case may not appear.
; Negation introduction: From $(p\; \backslash to\; q)$ and $(p\; \backslash to\; \backslash neg\; q)$, infer $\backslash neg\; p$.
: That is, $\backslash \; \backslash vdash\; \backslash neg\; p$.
; Negation elimination: From $\backslash neg\; p$, infer $(p\; \backslash to\; r)$.
: That is, $\backslash \; \backslash vdash\; (p\; \backslash to\; r)$.
; Double negation elimination
In propositional logic, double negation is the theorem that states that "If a statement is true, then it is not the case that the statement is not true." This is expressed by saying that a proposition ''A'' is logically equivalent to ''not (no ...

: From $\backslash neg\; \backslash neg\; p$, infer .
: That is, $\backslash neg\; \backslash neg\; p\; \backslash vdash\; p$.
; Conjunction introduction
Conjunction introduction (often abbreviated simply as conjunction and also called and introduction or adjunction) is a valid rule of inference of propositional logic. The rule makes it possible to introduce a conjunction into a logical proof ...

: From and , infer $(p\; \backslash land\; q)$.
: That is, $\backslash \; \backslash vdash\; (p\; \backslash land\; q)$.
; Conjunction elimination
In propositional logic, conjunction elimination (also called ''and'' elimination, ∧ elimination, or simplification)Hurley is a valid immediate inference, argument form and rule of inference which makes the inference that, if the conjunction ...

: From $(p\; \backslash land\; q)$, infer .
: From $(p\; \backslash land\; q)$, infer .
: That is, $(p\; \backslash land\; q)\; \backslash vdash\; p$ and $(p\; \backslash land\; q)\; \backslash vdash\; q$.
; Disjunction introduction: From , infer $(p\; \backslash lor\; q)$.
: From , infer $(p\; \backslash lor\; q)$.
: That is, $p\; \backslash vdash\; (p\; \backslash lor\; q)$ and $q\; \backslash vdash\; (p\; \backslash lor\; q)$.
; Disjunction elimination: From $(p\; \backslash lor\; q)$ and $(p\; \backslash to\; r)$ and $(q\; \backslash to\; r)$, infer .
: That is, $\backslash \; \backslash vdash\; r$.
; Biconditional introduction: From $(p\; \backslash to\; q)$ and $(q\; \backslash to\; p)$, infer $(p\; \backslash leftrightarrow\; q)$.
: That is, $\backslash \; \backslash vdash\; (p\; \backslash leftrightarrow\; q)$.
; Biconditional elimination: From $(p\; \backslash leftrightarrow\; q)$, infer $(p\; \backslash to\; q)$.
: From $(p\; \backslash leftrightarrow\; q)$, infer $(q\; \backslash to\; p)$.
: That is, $(p\; \backslash leftrightarrow\; q)\; \backslash vdash\; (p\; \backslash to\; q)$ and $(p\; \backslash leftrightarrow\; q)\; \backslash vdash\; (q\; \backslash to\; p)$.
;Modus ponens
In propositional logic, ''modus ponens'' (; MP), also known as ''modus ponendo ponens'' (Latin for "method of putting by placing") or implication elimination or affirming the antecedent, is a deductive argument form and rule of inference. ...

(conditional elimination) : From and $(p\; \backslash to\; q)$, infer .
: That is, $\backslash \; \backslash vdash\; q$.
; Conditional proof (conditional introduction) : From ccepting allows a proof of infer $(p\; \backslash to\; q)$.
: That is, $(p\; \backslash vdash\; q)\; \backslash vdash\; (p\; \backslash to\; q)$.
Basic and derived argument forms

Proofs in propositional calculus

One of the main uses of a propositional calculus, when interpreted for logical applications, is to determine relations of logical equivalence between propositional formulas. These relationships are determined by means of the available transformation rules, sequences of which are called ''derivations'' or ''proofs''. In the discussion to follow, a proof is presented as a sequence of numbered lines, with each line consisting of a single formula followed by a ''reason'' or ''justification'' for introducing that formula. Each premise of the argument, that is, an assumption introduced as an hypothesis of the argument, is listed at the beginning of the sequence and is marked as a "premise" in lieu of other justification. The conclusion is listed on the last line. A proof is complete if every line follows from the previous ones by the correct application of a transformation rule. (For a contrasting approach, see proof-trees).Example of a proof in natural deduction system

* To be shown that . * One possible proof of this (which, though valid, happens to contain more steps than are necessary) may be arranged as follows: Interpret $A\; \backslash vdash\; A$ as "Assuming , infer ". Read $\backslash vdash\; A\; \backslash to\; A$ as "Assuming nothing, infer that implies ", or "It is a tautology that implies ", or "It is always true that implies ".Example of a proof in a classical propositional calculus system

We now prove the same theorem $A\; \backslash to\; A$ in the axiomatic system bySoundness and completeness of the rules

The crucial properties of this set of rules are that they are ''sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by ...

'' and ''complete''. Informally this means that the rules are correct and that no other rules are required. These claims can be made more formal as follows.
Note that the proofs for the soundness and completeness of the propositional logic are not themselves proofs in propositional logic ; these are theorems in ZFC used as a metatheory
A metatheory or meta-theory is a theory whose subject matter is theory itself, aiming to describe existing theory in a systematic way. In mathematics and mathematical logic, a metatheory is a mathematical theory about another mathematical theory. ...

to prove properties of propositional logic.
We define a ''truth assignment'' as a function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-orient ...

that maps propositional variables to true or false. Informally such a truth assignment can be understood as the description of a possible state of affairs (or possible world
A possible world is a complete and consistent way the world is or could have been. Possible worlds are widely used as a formal device in logic, philosophy, and linguistics in order to provide a semantics for intensional and modal logic. Their me ...

) where certain statements are true and others are not. The semantics of formulas can then be formalized by defining for which "state of affairs" they are considered to be true, which is what is done by the following definition.
We define when such a truth assignment satisfies a certain if and only if
In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false.
The connective is bicond ...

* satisfies if and only if does not satisfy
* satisfies if and only if satisfies both and
* satisfies if and only if satisfies at least one of either or
* satisfies if and only if it is not the case that satisfies but not
* satisfies if and only if satisfies both and or satisfies neither one of them
With this definition we can now formalize what it means for a formula to be implied by a certain set of formulas. Informally this is true if in all worlds that are possible given the set of formulas the formula also holds. This leads to the following formal definition: We say that a set of well-formed formulas ''semantically entails'' (or ''implies'') a certain well-formed formula if all truth assignments that satisfy all the formulas in also satisfy .
Finally we define ''syntactical entailment'' such that is syntactically entailed by if and only if we can derive it with the inference rules that were presented above in a finite number of steps. This allows us to formulate exactly what it means for the set of inference rules to be sound and complete:
Soundness: If the set of well-formed formulas ''syntactically'' entails the well-formed formula then ''semantically'' entails .
Completeness: If the set of well-formed formulas ''semantically'' entails the well-formed formula then ''syntactically'' entails .
For the above set of rules this is indeed the case.
Sketch of a soundness proof

(For mostlogical system
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system.
A for ...

s, this is the comparatively "simple" direction of proof)
Notational conventions: Let be a variable ranging over sets of sentences. Let and range over sentences. For " syntactically entails " we write " proves ". For " semantically entails " we write " implies ".
We want to show: (if proves , then implies ).
We note that " proves " has an inductive definition, and that gives us the immediate resources for demonstrating claims of the form "If proves , then ...". So our proof proceeds by induction.
Notice that Basis Step II can be omitted for natural deduction systems because they have no axioms. When used, Step II involves showing that each of the axioms is a (semantic) logical truth
Logical truth is one of the most fundamental concepts in logic. Broadly speaking, a logical truth is a statement which is true regardless of the truth or falsity of its constituent propositions. In other words, a logical truth is a statement whic ...

.
The Basis steps demonstrate that the simplest provable sentences from are also implied by , for any . (The proof is simple, since the semantic fact that a set implies any of its members, is also trivial.) The Inductive step will systematically cover all the further sentences that might be provable—by considering each case where we might reach a logical conclusion using an inference rule—and shows that if a new sentence is provable, it is also logically implied. (For example, we might have a rule telling us that from "" we can derive " or ". In III.a We assume that if is provable it is implied. We also know that if is provable then " or " is provable. We have to show that then " or " too is implied. We do so by appeal to the semantic definition and the assumption we just made. is provable from , we assume. So it is also implied by . So any semantic valuation making all of true makes true. But any valuation making true makes " or " true, by the defined semantics for "or". So any valuation which makes all of true makes " or " true. So " or " is implied.) Generally, the Inductive step will consist of a lengthy but simple case-by-case analysis of all the rules of inference, showing that each "preserves" semantic implication.
By the definition of provability, there are no sentences provable other than by being a member of , an axiom, or following by a rule; so if all of those are semantically implied, the deduction calculus is sound.
Sketch of completeness proof

(This is usually the much harder direction of proof.) We adopt the same notational conventions as above. We want to show: If implies , then proves . We proceed bycontraposition
In logic and mathematics, contraposition refers to the inference of going from a conditional statement into its logically equivalent contrapositive, and an associated proof method known as proof by contraposition. The contrapositive of a statem ...

: We show instead that if does not prove then does not imply . If we show that there is a model
A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure.
Models c ...

where does not hold despite being true, then obviously does not imply . The idea is to build such a model out of our very assumption that does not prove .
Thus every system that has modus ponens as an inference rule, and proves the following theorems (including substitutions thereof) is complete:
* $p\; \backslash to\; (\backslash neg\; p\; \backslash to\; q)$
* $(p\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; p\; \backslash to\; q)\; \backslash to\; q)$
* $p\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q))$
* $p\; \backslash to\; (\backslash neg\; q\; \backslash to\; \backslash neg\; (p\; \backslash to\; q))$
* $\backslash neg\; p\; \backslash to\; (p\; \backslash to\; q)$
* $p\; \backslash to\; p$
* $p\; \backslash to\; (q\; \backslash to\; p)$
* $(p\; \backslash to\; (q\; \backslash to\; r))\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; (p\; \backslash to\; r))$
The first five are used for the satisfaction of the five conditions in stage III above, and the last three for proving the deduction theorem.
Example

As an example, it can be shown that as any other tautology, the three axioms of the classical propositional calculus system described earlier can be proven in any system that satisfies the above, namely that has modus ponens as an inference rule, and proves the above eight theorems (including substitutions thereof). Out of the eight theorems, the last two are two of the three axioms; the third axiom, $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (p\; \backslash to\; q)$, can be proven as well, as we now show. For the proof we may use the hypothetical syllogism theorem (in the form relevant for this axiomatic system), since it only relies on the two axioms that are already in the above set of eight theorems. The proof then is as follows: # $q\; \backslash to\; (p\; \backslash to\; q)$ (instance of the 7th theorem) # $(q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q)))$ (instance of the 7th theorem) # $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q))$ (from (1) and (2) by modus ponens) # $(\backslash neg\; p\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (\backslash neg\; q\backslash to\; (p\backslash to\; q)))$ (instance of the hypothetical syllogism theorem) # $(\backslash neg\; p\; \backslash to\; (p\; \backslash to\; q))$ (instance of the 5th theorem) # $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (\backslash neg\; q\backslash to\; (p\backslash to\; q))$ (from (5) and (4) by modus ponens) # $(q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q))$ (instance of the 2nd theorem) # $((q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q))\; )\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q))))$ (instance of the 7th theorem) # $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q)))$ (from (7) and (8) by modus ponens) # $((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q))))\; \backslash to$ #:: $(((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q)))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q))))$ (instance of the 8th theorem) # $((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q)))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q)))$ (from (9) and (10) by modus ponens) # $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q))$ (from (3) and (11) by modus ponens) # $((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; ((\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; q)))\; \backslash to\; (((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q)))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (p\; \backslash to\; q)))$ (instance of the 8th theorem) # $((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (\backslash neg\; q\; \backslash to\; (p\; \backslash to\; q)))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (p\; \backslash to\; q))$ (from (12) and (13) by modus ponens) # $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (p\; \backslash to\; q)$ (from (6) and (14) by modus ponens)Verifying completeness for the classical propositional calculus system

We now verify that the classical propositional calculus system described earlier can indeed prove the required eight theorems mentioned above. We use several lemmas provenhere
Here is an adverb that means "in, on, or at this place". It may also refer to:
Software
* Here Technologies, a mapping company
* Here WeGo (formerly Here Maps), a mobile app and map website by Here
Television
* Here TV (formerly "here!"), a ...

:
: (DN1) $\backslash neg\; \backslash neg\; p\; \backslash to\; p$ - Double negation (one direction)
: (DN2) $p\; \backslash to\; \backslash neg\; \backslash neg\; p$ - Double negation (another direction)
: (HS1) $(q\; \backslash to\; r)\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; (p\; \backslash to\; r))$ - one form of Hypothetical syllogism
In classical logic, a hypothetical syllogism is a valid argument form, a syllogism with a conditional statement for one or both of its premises.
An example in English:
:If I do not wake up, then I cannot go to work.
:If I cannot go to work, then ...

: (HS2) $(p\; \backslash to\; q)\; \backslash to\; ((q\; \backslash to\; r)\; \backslash to\; (p\; \backslash to\; r))$ - another form of Hypothetical syllogism
: (TR1) $(p\; \backslash to\; q)\; \backslash to\; (\backslash neg\; q\; \backslash to\; \backslash neg\; p)$ - Transposition
:(TR2) $(\backslash neg\; p\; \backslash to\; q)\; \backslash to\; (\backslash neg\; q\; \backslash to\; p)$ - another form of transposition.
:(L1) $p\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; q)$
:(L3) $(\backslash neg\; p\; \backslash to\; p)\; \backslash to\; p$
We also use the method of the hypothetical syllogism metatheorem as a shorthand for several proof steps.
* $p\; \backslash to\; (\backslash neg\; p\; \backslash to\; q)$ - proof:
*# $p\; \backslash to\; (\backslash neg\; q\; \backslash to\; p)$ (instance of (A1))
*# $(\backslash neg\; q\; \backslash to\; p)\; \backslash to\; (\backslash neg\; p\; \backslash to\; \backslash neg\backslash neg\; q)$ (instance of (TR1))
*# $p\; \backslash to\; (\backslash neg\; p\; \backslash to\; \backslash neg\backslash neg\; q)$ (from (1) and (2) using the hypothetical syllogism metatheorem)
*# $\backslash neg\backslash neg\; q\; \backslash to\; q$ (instance of (DN1))
*# $(\backslash neg\backslash neg\; q\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; p\; \backslash to\; \backslash neg\backslash neg\; q)\; \backslash to\; (\backslash neg\; p\; \backslash to\; q))$ (instance of (HS1))
*# $(\backslash neg\; p\; \backslash to\; \backslash neg\backslash neg\; q)\; \backslash to\; (\backslash neg\; p\; \backslash to\; q)$ (from (4) and (5) using modus ponens)
*# $p\; \backslash to\; (\backslash neg\; p\; \backslash to\; q)$ (from (3) and (6) using the hypothetical syllogism metatheorem)
* $(p\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; p\; \backslash to\; q)\; \backslash to\; q)$ - proof:
*# $(p\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; (\backslash neg\; q\; \backslash to\; q))$ (instance of (HS1))
*# $(\backslash neg\; q\; \backslash to\; q)\; \backslash to\; q$ (instance of (L3))
*# $((\backslash neg\; q\; \backslash to\; q)\; \backslash to\; q)\; \backslash to\; (((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; (\backslash neg\; q\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; q))$ (instance of (HS1))
*# $((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; (\backslash neg\; q\; \backslash to\; q))\; \backslash to\; ((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; q)$ (from (2) and (3) by modus ponens)
*# $(p\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; q)$ (from (1) and (4) using the hypothetical syllogism metatheorem)
*# $(\backslash neg\; p\; \backslash to\; q)\; \backslash to\; (\backslash neg\; q\; \backslash to\; p)$ (instance of (TR2))
*# $((\backslash neg\; p\; \backslash to\; q)\; \backslash to\; (\backslash neg\; q\; \backslash to\; p))\; \backslash to\; (((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; p\; \backslash to\; q)\; \backslash to\; q))$ (instance of (HS2))
*# $((\backslash neg\; q\; \backslash to\; p)\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; p\; \backslash to\; q)\; \backslash to\; q)$ (from (6) and (7) using modus ponens)
*# $(p\; \backslash to\; q)\; \backslash to\; ((\backslash neg\; p\; \backslash to\; q)\; \backslash to\; q)$ (from (5) and (8) using the hypothetical syllogism metatheorem)
* $p\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q))$ - proof:
*# $q\; \backslash to\; (p\; \backslash to\; q)$ (instance of (A1))
*# $(q\; \backslash to\; (p\; \backslash to\; q))\; \backslash to\; (p\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q)))$ (instance of (A1))
*# $p\; \backslash to\; (q\; \backslash to\; (p\; \backslash to\; q))$ (from (1) and (2) using modus ponens)
* $p\; \backslash to\; (\backslash neg\; q\; \backslash to\; \backslash neg\; (p\; \backslash to\; q))$ - proof:
*# $p\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; q)$ (instance of (L1))
*# $((p\; \backslash to\; q)\; \backslash to\; q)\; \backslash to\; (\backslash neg\; q\; \backslash to\; \backslash neg\; (p\; \backslash to\; q))$ (instance of (TR1))
*# $p\; \backslash to\; (\backslash neg\; q\; \backslash to\; \backslash neg\; (p\; \backslash to\; q))$ (from (1) and (2) using the hypothetical syllogism metatheorem)
* $\backslash neg\; p\; \backslash to\; (p\; \backslash to\; q)$ - proof:
*# $\backslash neg\; p\; \backslash to\; (\backslash neg\; q\; \backslash to\; \backslash neg\; p)$ (instance of (A1))
*# $(\backslash neg\; q\; \backslash to\; \backslash neg\; p)\; \backslash to\; (p\; \backslash to\; q)$ (instance of (A3))
*# $\backslash neg\; p\; \backslash to\; (p\; \backslash to\; q)$ (from (1) and (2) using the hypothetical syllogism metatheorem)
* $p\; \backslash to\; p$ - proof given in the proof example above
* $p\; \backslash to\; (q\; \backslash to\; p)$ - axiom (A1)
* $(p\; \backslash to\; (q\; \backslash to\; r))\; \backslash to\; ((p\; \backslash to\; q)\; \backslash to\; (p\; \backslash to\; r))$ - axiom (A2)
Another outline for a completeness proof

If a formula is a tautology, then there is atruth table
A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional argumen ...

for it which shows that each valuation yields the value true for the formula. Consider such a valuation. By mathematical induction on the length of the subformulas, show that the truth or falsity of the subformula follows from the truth or falsity (as appropriate for the valuation) of each propositional variable in the subformula. Then combine the lines of the truth table together two at a time by using "( is true implies ) implies (( is false implies ) implies )". Keep repeating this until all dependencies on propositional variables have been eliminated. The result is that we have proved the given tautology. Since every tautology is provable, the logic is complete.
Interpretation of a truth-functional propositional calculus

An interpretation of a truth-functional propositional calculus $\backslash mathcal$ is an assignment to each propositional symbol of $\backslash mathcal$ of one or the other (but not both) of thetruth value
In logic and mathematics, a truth value, sometimes called a logical value, is a value indicating the relation of a proposition to truth, which in classical logic has only two possible values (''true'' or ''false'').
Computing
In some program ...

s truth
Truth is the property of being in accord with fact or reality.Merriam-Webster's Online Dictionarytruth 2005 In everyday language, truth is typically ascribed to things that aim to represent reality or otherwise correspond to it, such as beliefs, ...

(T) and falsity
Deception or falsehood is an act or statement that misleads, hides the truth, or promotes a belief, concept, or idea that is not true. It is often done for personal gain or advantage. Deception can involve dissimulation, propaganda and sleight o ...

(F), and an assignment to the connective symbols of $\backslash mathcal$ of their usual truth-functional meanings. An interpretation of a truth-functional propositional calculus may also be expressed in terms of truth tables
A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional argume ...

.
For $n$ distinct propositional symbols there are $2^n$ distinct possible interpretations. For any particular symbol $a$, for example, there are $2^1=2$ possible interpretations:
# $a$ is assigned T, or
# $a$ is assigned F.
For the pair $a$, $b$ there are $2^2=4$ possible interpretations:
# both are assigned T,
# both are assigned F,
# $a$ is assigned T and $b$ is assigned F, or
# $a$ is assigned F and $b$ is assigned T.
Since $\backslash mathcal$ has $\backslash aleph\_0$, that is, denumerably many propositional symbols, there are $2^=\backslash mathfrak\; c$, and therefore uncountably many distinct possible interpretations of $\backslash mathcal$.
Interpretation of a sentence of truth-functional propositional logic

If and are formulas of $\backslash mathcal$ and $\backslash mathcal$ is an interpretation of $\backslash mathcal$ then the following definitions apply: * A sentence of propositional logic is ''true under an interpretation'' $\backslash mathcal$ if $\backslash mathcal$ assigns the truth value T to that sentence. If a sentence istrue
True most commonly refers to truth, the state of being in congruence with fact or reality.
True may also refer to:
Places
* True, West Virginia, an unincorporated community in the United States
* True, Wisconsin, a town in the United States
* ...

under an interpretation, then that interpretation is called a ''model'' of that sentence.
* is ''false under an interpretation'' $\backslash mathcal$ if is not true under $\backslash mathcal$.
* A sentence of propositional logic is ''logically valid'' if it is true under every interpretation.
*: $\backslash models$ means that is logically valid.
* A sentence of propositional logic is a '' semantic consequence'' of a sentence if there is no interpretation under which is true and is false.
* A sentence of propositional logic is ''consistent
In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent ...

'' if it is true under at least one interpretation. It is inconsistent if it is not consistent.
Some consequences of these definitions:
* For any given interpretation a given formula is either true or false.
* No formula is both true and false under the same interpretation.
* is false for a given interpretation $\backslash neg\backslash phi$ is true for that interpretation; and is true under an interpretation $\backslash neg\backslash phi$ is false under that interpretation.
* If and $(\backslash phi\; \backslash to\; \backslash psi)$ are both true under a given interpretation, then is true under that interpretation.
* If $\backslash models\_\backslash phi$ and $\backslash models\_(\backslash phi\; \backslash to\; \backslash psi)$, then $\backslash models\_\backslash psi$.
* $\backslash neg\backslash phi$ is true under $\backslash mathcal$ is not true under $\backslash mathcal$.
* $(\backslash phi\; \backslash to\; \backslash psi)$ is true under $\backslash mathcal$ either is not true under $\backslash mathcal$ or is true under $\backslash mathcal$.
* A sentence of propositional logic is a semantic consequence of a sentence $(\backslash phi\; \backslash to\; \backslash psi)$ is logically valid, that is, $\backslash phi\; \backslash models\_\; \backslash psi$ $\backslash models\_(\backslash phi\; \backslash to\; \backslash psi)$.
Alternative calculus

It is possible to define another version of propositional calculus, which defines most of the syntax of the logical operators by means of axioms, and which uses only one inference rule.Axioms

Let , , and stand for well-formed formulas. (The well-formed formulas themselves would not contain any Greek letters, but only capital Roman letters, connective operators, and parentheses.) Then the axioms are as follows: *Axiom may be considered to be a "distributive property of implication with respect to implication." *Axioms and correspond to "conjunction elimination". The relation between and reflects the commutativity of the conjunction operator. *Axiom corresponds to "conjunction introduction." *Axioms and correspond to "disjunction introduction." The relation between and reflects the commutativity of the disjunction operator. *Axiom corresponds to "reductio ad absurdum." *Axiom says that "anything can be deduced from a contradiction." *Axiom is called " tertium non-datur" (Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of ...

: "a third is not given") and reflects the semantic valuation of propositional formulas: a formula can have a truth-value of either true or false. There is no third truth-value, at least not in classical logic. Intuitionistic logic
Intuitionistic logic, sometimes more generally called constructive logic, refers to systems of symbolic logic that differ from the systems used for classical logic by more closely mirroring the notion of constructive proof. In particular, systems ...

ians do not accept the axiom .
Inference rule

The inference rule isMeta-inference rule

Let a demonstration be represented by a sequence, with hypotheses to the left of theturnstile
A turnstile (also called a turnpike, gateline, baffle gate, automated gate, turn gate in some regions) is a form of gate which allows one person to pass at a time. A turnstile can be configured to enforce one-way human traffic. In addition, a t ...

and the conclusion to the right of the turnstile. Then the deduction theorem In mathematical logic, a deduction theorem is a metatheorem that justifies doing conditional proofs—to prove an implication ''A'' → ''B'', assume ''A'' as an hypothesis and then proceed to derive ''B''—in systems that do not have a ...

can be stated as follows:
: ''If the sequence''
::$\backslash phi\_1,\; \backslash \; \backslash phi\_2,\; \backslash \; ...\; ,\; \backslash \; \backslash phi\_n,\; \backslash \; \backslash chi\; \backslash vdash\; \backslash psi$
: ''has been demonstrated, then it is also possible to demonstrate the sequence''
::$\backslash phi\_1,\; \backslash \; \backslash phi\_2,\; \backslash \; ...,\; \backslash \; \backslash phi\_n\; \backslash vdash\; \backslash chi\; \backslash to\; \backslash psi$.
This deduction theorem (DT) is not itself formulated with propositional calculus: it is not a theorem of propositional calculus, but a theorem about propositional calculus. In this sense, it is a meta-theorem, comparable to theorems about the soundness or completeness of propositional calculus.
On the other hand, DT is so useful for simplifying the syntactical proof process that it can be considered and used as another inference rule, accompanying modus ponens. In this sense, DT corresponds to the natural conditional proof inference rule which is part of the first version of propositional calculus introduced in this article.
The converse of DT is also valid:
: ''If the sequence''
::$\backslash phi\_1,\; \backslash \; \backslash phi\_2,\; \backslash \; ...,\; \backslash \; \backslash phi\_n\; \backslash vdash\; \backslash chi\; \backslash to\; \backslash psi$
: ''has been demonstrated, then it is also possible to demonstrate the sequence''
::$\backslash phi\_1,\; \backslash \; \backslash phi\_2,\; \backslash \; ...\; ,\; \backslash \; \backslash phi\_n,\; \backslash \; \backslash chi\; \backslash vdash\; \backslash psi$
in fact, the validity of the converse of DT is almost trivial compared to that of DT:
: ''If''
:: $\backslash phi\_1,\; \backslash \; ...\; ,\; \backslash \; \backslash phi\_n\; \backslash vdash\; \backslash chi\; \backslash to\; \backslash psi$
: ''then''
:: 1: $\backslash phi\_1,\; \backslash \; ...\; ,\; \backslash \; \backslash phi\_n,\; \backslash \; \backslash chi\; \backslash vdash\; \backslash chi\; \backslash to\; \backslash psi$
:: 2: $\backslash phi\_1,\; \backslash \; ...\; ,\; \backslash \; \backslash phi\_n,\; \backslash \; \backslash chi\; \backslash vdash\; \backslash chi$
: ''and from (1) and (2) can be deduced''
:: 3: $\backslash phi\_1,\; \backslash \; ...\; ,\; \backslash \; \backslash phi\_n,\; \backslash \; \backslash chi\; \backslash vdash\; \backslash psi$
: ''by means of modus ponens, Q.E.D.''
The converse of DT has powerful implications: it can be used to convert an axiom into an inference rule. For example, by axiom AND-1 we have,
: $\backslash vdash\; \backslash phi\; \backslash wedge\; \backslash chi\; \backslash to\; \backslash phi,$
which can be transformed by means of the converse of the deduction theorem into
: $\backslash phi\; \backslash wedge\; \backslash chi\; \backslash vdash\; \backslash phi,$
which tells us that the inference rule
: $\backslash frac$
is admissible. This inference rule is conjunction elimination
In propositional logic, conjunction elimination (also called ''and'' elimination, ∧ elimination, or simplification)Hurley is a valid immediate inference, argument form and rule of inference which makes the inference that, if the conjunction ...

, one of the ten inference rules used in the first version (in this article) of the propositional calculus.
Example of a proof

The following is an example of a (syntactical) demonstration, involving only axioms and : Prove: $A\; \backslash to\; A$ (Reflexivity of implication). Proof: # $(A\; \backslash to\; ((B\; \backslash to\; A)\; \backslash to\; A))\; \backslash to\; ((A\; \backslash to\; (B\; \backslash to\; A))\; \backslash to\; (A\; \backslash to\; A))$ #: Axiom with $\backslash phi\; =\; A,\; \backslash chi\; =\; B\; \backslash to\; A,\; \backslash psi\; =\; A$ # $A\; \backslash to\; ((B\; \backslash to\; A)\; \backslash to\; A)$ #: Axiom with $\backslash phi\; =\; A,\; \backslash chi\; =\; B\; \backslash to\; A$ # $(A\; \backslash to\; (B\; \backslash to\; A))\; \backslash to\; (A\; \backslash to\; A)$ #: From (1) and (2) by modus ponens. # $A\; \backslash to\; (B\; \backslash to\; A)$ #: Axiom with $\backslash phi\; =\; A,\; \backslash chi\; =\; B$ # $A\; \backslash to\; A$ #: From (3) and (4) by modus ponens.Equivalence to equational logics

The preceding alternative calculus is an example of a Hilbert-style deduction system. In the case of propositional systems the axioms are terms built with logical connectives and the only inference rule is modus ponens. Equational logic as standardly used informally in high school algebra is a different kind of calculus from Hilbert systems. Its theorems are equations and its inference rules express the properties of equality, namely that it is a congruence on terms that admits substitution. Classical propositional calculus as described above is equivalent toBoolean algebra
In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas i ...

, while intuitionistic propositional calculus is equivalent to Heyting algebra In mathematics, a Heyting algebra (also known as pseudo-Boolean algebra) is a bounded lattice (with join and meet operations written ∨ and ∧ and with least element 0 and greatest element 1) equipped with a binary operation ''a'' → ''b'' of '' ...

. The equivalence is shown by translation in each direction of the theorems of the respective systems. Theorems $\backslash phi$ of classical or intuitionistic propositional calculus are translated as equations $\backslash phi\; =\; 1$ of Boolean or Heyting algebra respectively. Conversely theorems $x\; =\; y$ of Boolean or Heyting algebra are translated as theorems $(x\; \backslash to\; y)\; \backslash land\; (y\; \backslash to\; x)$ of classical or intuitionistic calculus respectively, for which $x\; \backslash equiv\; y$ is a standard abbreviation. In the case of Boolean algebra $x\; =\; y$ can also be translated as $(x\; \backslash land\; y)\; \backslash lor\; (\backslash neg\; x\; \backslash land\; \backslash neg\; y)$, but this translation is incorrect intuitionistically.
In both Boolean and Heyting algebra, inequality $x\; \backslash le\; y$ can be used in place of equality. The equality $x\; =\; y$ is expressible as a pair of inequalities $x\; \backslash le\; y$ and $y\; \backslash le\; x$. Conversely the inequality $x\; \backslash le\; y$ is expressible as the equality $x\; \backslash land\; y\; =\; x$, or as $x\; \backslash lor\; y\; =\; y$. The significance of inequality for Hilbert-style systems is that it corresponds to the latter's deduction or entailment
Logical consequence (also entailment) is a fundamental concept in logic, which describes the relationship between statements that hold true when one statement logically ''follows from'' one or more statements. A valid logical argument is one ...

symbol $\backslash vdash$. An entailment
::$\backslash phi\_1,\; \backslash \; \backslash phi\_2,\; \backslash \; \backslash dots,\; \backslash \; \backslash phi\_n\; \backslash vdash\; \backslash psi$
is translated in the inequality version of the algebraic framework as
::$\backslash phi\_1\backslash \; \backslash land\backslash \; \backslash phi\_2\backslash \; \backslash land\backslash \; \backslash dots\backslash \; \backslash land\; \backslash \; \backslash phi\_n\backslash \; \backslash \; \backslash le\backslash \; \backslash \; \backslash psi$
Conversely the algebraic inequality $x\; \backslash le\; y$ is translated as the entailment
::$x\backslash \; \backslash vdash\backslash \; y$.
The difference between implication $x\; \backslash to\; y$ and inequality or entailment
Logical consequence (also entailment) is a fundamental concept in logic, which describes the relationship between statements that hold true when one statement logically ''follows from'' one or more statements. A valid logical argument is one ...

$x\; \backslash le\; y$ or $x\backslash \; \backslash vdash\backslash \; y$ is that the former is internal to the logic while the latter is external. Internal implication between two terms is another term of the same kind. Entailment as external implication between two terms expresses a metatruth outside the language of the logic, and is considered part of the metalanguage. Even when the logic under study is intuitionistic, entailment is ordinarily understood classically as two-valued: either the left side entails, or is less-or-equal to, the right side, or it is not.
Similar but more complex translations to and from algebraic logics are possible for natural deduction systems as described above and for the sequent calculus
In mathematical logic, sequent calculus is a style of formal logical argumentation in which every line of a proof is a conditional tautology (called a sequent by Gerhard Gentzen) instead of an unconditional tautology. Each conditional tautology ...

. The entailments of the latter can be interpreted as two-valued, but a more insightful interpretation is as a set, the elements of which can be understood as abstract proofs organized as the morphisms of a category
Category, plural categories, may refer to:
Philosophy and general uses
*Categorization, categories in cognitive science, information science and generally
*Category of being
* ''Categories'' (Aristotle)
* Category (Kant)
* Categories (Peirce)
...

. In this interpretation the cut rule of the sequent calculus corresponds to composition in the category. Boolean and Heyting algebras enter this picture as special categories having at most one morphism per homset, i.e., one proof per entailment, corresponding to the idea that existence of proofs is all that matters: any proof will do and there is no point in distinguishing them.
Graphical calculi

It is possible to generalize the definition of a formal language from a set of finite sequences over a finite basis to include many other sets of mathematical structures, so long as they are built up by finitary means from finite materials. What's more, many of these families of formal structures are especially well-suited for use in logic. For example, there are many families ofgraphs
Graph may refer to:
Mathematics
* Graph (discrete mathematics), a structure made of vertices and edges
**Graph theory, the study of such graphs and their properties
* Graph (topology), a topological space resembling a graph in the sense of disc ...

that are close enough analogues of formal languages that the concept of a calculus is quite easily and naturally extended to them. Many species of graphs arise as '' parse graphs'' in the syntactic analysis of the corresponding families of text structures. The exigencies of practical computation on formal languages frequently demand that text strings be converted into pointer structure renditions of parse graphs, simply as a matter of checking whether strings are well-formed formulas or not. Once this is done, there are many advantages to be gained from developing the graphical analogue of the calculus on strings. The mapping from strings to parse graphs is called ''parsing
Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar. The term ''parsing'' comes from Latin ...

'' and the inverse mapping from parse graphs to strings is achieved by an operation that is called '' traversing'' the graph.
Other logical calculi

Propositional calculus is about the simplest kind of logical calculus in current use. It can be extended in several ways. ( Aristotelian "syllogistic" calculus, which is largely supplanted in modern logic, is in ''some'' ways simpler – but in other ways more complex – than propositional calculus.) The most immediate way to develop a more complex logical calculus is to introduce rules that are sensitive to more fine-grained details of the sentences being used.First-order logic
First-order logic—also known as predicate logic, quantificational logic, and first-order predicate calculus—is a collection of formal systems used in mathematics, philosophy, linguistics, and computer science. First-order logic uses quantifie ...

(a.k.a. first-order predicate logic) results when the "atomic sentences" of propositional logic are broken up into terms, variables, predicates, and quantifiers, all keeping the rules of propositional logic with some new ones introduced. (For example, from "All dogs are mammals" we may infer "If Rover is a dog then Rover is a mammal".) With the tools of first-order logic it is possible to formulate a number of theories, either with explicit axioms or by rules of inference, that can themselves be treated as logical calculi. Arithmetic
Arithmetic () is an elementary part of mathematics that consists of the study of the properties of the traditional operations on numbers— addition, subtraction, multiplication, division, exponentiation, and extraction of roots. In the 19t ...

is the best known of these; others include set theory
Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly concer ...

and mereology
In logic, philosophy and related fields, mereology ( (root: , ''mere-'', 'part') and the suffix ''-logy'', 'study, discussion, science') is the study of parts and the wholes they form. Whereas set theory is founded on the membership relation bet ...

. Second-order logic
In logic and mathematics, second-order logic is an extension of first-order logic, which itself is an extension of propositional logic. Second-order logic is in turn extended by higher-order logic and type theory.
First-order logic quantifies on ...

and other higher-order logic
mathematics and logic, a higher-order logic is a form of predicate logic that is distinguished from first-order logic by additional quantifiers and, sometimes, stronger semantics. Higher-order logics with their standard semantics are more expres ...

s are formal extensions of first-order logic. Thus, it makes sense to refer to propositional logic as ''"zeroth-order logic"'', when comparing it with these logics.
Modal logic
Modal logic is a collection of formal systems developed to represent statements about necessity and possibility. It plays a major role in philosophy of language, epistemology, metaphysics, and natural language semantics. Modal logics extend other ...

also offers a variety of inferences that cannot be captured in propositional calculus. For example, from "Necessarily " we may infer that . From we may infer "It is possible that ". The translation between modal logics and algebraic logics concerns classical and intuitionistic logics but with the introduction of a unary operator on Boolean or Heyting algebras, different from the Boolean operations, interpreting the possibility modality, and in the case of Heyting algebra a second operator interpreting necessity (for Boolean algebra this is redundant since necessity is the De Morgan dual of possibility). The first operator preserves 0 and disjunction while the second preserves 1 and conjunction.
Many-valued logic
Many-valued logic (also multi- or multiple-valued logic) refers to a propositional calculus in which there are more than two truth values. Traditionally, in Aristotle's logical calculus, there were only two possible values (i.e., "true" and "false ...

s are those allowing sentences to have values other than ''true'' and ''false''. (For example, ''neither'' and ''both'' are standard "extra values"; "continuum logic" allows each sentence to have any of an infinite number of "degrees of truth" between ''true'' and ''false''.) These logics often require calculational devices quite distinct from propositional calculus. When the values form a Boolean algebra (which may have more than two or even infinitely many values), many-valued logic reduces to classical logic; many-valued logics are therefore only of independent interest when the values form an algebra that is not Boolean.
Solvers

Deciding satisfiability of propositional logic formulas is anNP-complete
In computational complexity theory, a problem is NP-complete when:
# it is a problem for which the correctness of each solution can be verified quickly (namely, in polynomial time) and a brute-force search algorithm can find a solution by trying ...

problem. However, practical methods exist (e.g., DPLL algorithm
In logic and computer science, the Davis–Putnam–Logemann–Loveland (DPLL) algorithm is a complete, backtracking-based search algorithm for deciding the satisfiability of propositional logic formulae in conjunctive normal form, i.e. for solv ...

, 1962; Chaff algorithm, 2001) that are very fast for many useful cases. Recent work has extended the SAT solver
The SAT ( ) is a standardized test widely used for college admissions in the United States. Since its debut in 1926, its name and scoring have changed several times; originally called the Scholastic Aptitude Test, it was later called the Schol ...

algorithms to work with propositions containing arithmetic expressions; these are the SMT solvers.
See also

Higher logical levels

*First-order logic
First-order logic—also known as predicate logic, quantificational logic, and first-order predicate calculus—is a collection of formal systems used in mathematics, philosophy, linguistics, and computer science. First-order logic uses quantifie ...

* Second-order propositional logic
* Second-order logic
In logic and mathematics, second-order logic is an extension of first-order logic, which itself is an extension of propositional logic. Second-order logic is in turn extended by higher-order logic and type theory.
First-order logic quantifies on ...

* Higher-order logic
mathematics and logic, a higher-order logic is a form of predicate logic that is distinguished from first-order logic by additional quantifiers and, sometimes, stronger semantics. Higher-order logics with their standard semantics are more expres ...

Related topics

*Boolean algebra (logic)
In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas i ...

* Boolean algebra (structure)
In abstract algebra, a Boolean algebra or Boolean lattice is a complemented distributive lattice. This type of algebraic structure captures essential properties of both set operations and logic operations. A Boolean algebra can be seen as a gen ...

* Boolean algebra topics
* Boolean domain
In mathematics and abstract algebra, a Boolean domain is a set consisting of exactly two elements whose interpretations include ''false'' and ''true''. In logic, mathematics and theoretical computer science, a Boolean domain is usually written as ...

* Boolean function
In mathematics, a Boolean function is a function whose arguments and result assume values from a two-element set (usually , or ). Alternative names are switching function, used especially in older computer science literature, and truth function ...

* Boolean-valued function
* Categorical logic
__NOTOC__
Categorical logic is the branch of mathematics in which tools and concepts from category theory are applied to the study of mathematical logic. It is also notable for its connections to theoretical computer science.
In broad terms, categ ...

* Combinational logic
In automata theory, combinational logic (also referred to as time-independent logic or combinatorial logic) is a type of digital logic which is implemented by Boolean circuits, where the output is a pure function of the present input only. This ...

* Combinatory logic
Combinatory logic is a notation to eliminate the need for quantified variables in mathematical logic. It was introduced by Moses Schönfinkel and Haskell Curry, and has more recently been used in computer science as a theoretical model of compu ...

* Conceptual graph
A conceptual graph (CG) is a formalism for knowledge representation. In the first published paper on CGs, John F. Sowa used them to represent the conceptual schemas used in database systems. The first book on CGs applied them to a wide range of ...

* Disjunctive syllogism
* Entitative graph
* Equational logic
* Existential graph
An existential graph is a type of diagrammatic or visual notation for logical expressions, proposed by Charles Sanders Peirce, who wrote on graphical logic as early as 1882,Peirce, C. S., " n Junctures and Fractures in Logic (editors' title for ...

* Frege's propositional calculus
* Implicational propositional calculus
* Intuitionistic propositional calculus
* Jean Buridan
* ''Laws of Form
''Laws of Form'' (hereinafter ''LoF'') is a book by G. Spencer-Brown, published in 1969, that straddles the boundary between mathematics and philosophy. ''LoF'' describes three distinct logical systems:
* The "primary arithmetic" (described in C ...

''
* List of logic symbols
In logic, a set of symbols is commonly used to express logical representation. The following table lists many common symbols, together with their name, how they should be read out loud, and the related field of mathematics. Additionally, the subs ...

* Logical graph
* Logical NOR
* Logical value
In logic and mathematics, a truth value, sometimes called a logical value, is a value indicating the relation of a proposition to truth, which in classical logic has only two possible values ('' true'' or '' false'').
Computing
In some progr ...

* Mathematical logic
Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical properties of formal ...

* Operation (mathematics)
In mathematics, an operation is a function which takes zero or more input values (also called "''operands''" or "arguments") to a well-defined output value. The number of operands is the arity of the operation.
The most commonly studied operati ...

* Paul of Venice
* Peirce's law
* Peter of Spain (author)
* Propositional formula
In propositional logic, a propositional formula is a type of syntactic formula which is well formed and has a truth value. If the values of all variables in a propositional formula are given, it determines a unique truth value. A propositional for ...

* Symmetric difference
In mathematics, the symmetric difference of two sets, also known as the disjunctive union, is the set of elements which are in either of the sets, but not in their intersection. For example, the symmetric difference of the sets \ and \ is \.
Th ...

* Tautology (rule of inference)
* Truth function
In logic, a truth function is a function that accepts truth values as input and produces a unique truth value as output. In other words: The input and output of a truth function are all truth values; a truth function will always output exactly on ...

* Truth table
A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional argumen ...

* Walter Burley
* William of Sherwood
References

Further reading

* Brown, Frank Markham (2003), ''Boolean Reasoning: The Logic of Boolean Equations'', 1st edition, Kluwer Academic Publishers, Norwell, MA. 2nd edition, Dover Publications, Mineola, NY. * Chang, C.C. and Keisler, H.J. (1973), ''Model Theory'', North-Holland, Amsterdam, Netherlands. * Kohavi, Zvi (1978), ''Switching and Finite Automata Theory'', 1st edition, McGraw–Hill, 1970. 2nd edition, McGraw–Hill, 1978. * Korfhage, Robert R. (1974), ''Discrete Computational Structures'', Academic Press, New York, NY. * Lambek, J. and Scott, P.J. (1986), ''Introduction to Higher Order Categorical Logic'', Cambridge University Press, Cambridge, UK. * Mendelson, Elliot (1964), ''Introduction to Mathematical Logic'', D. Van Nostrand Company.Related works

*External links

* Klement, Kevin C. (2006), "Propositional Logic", in James Fieser and Bradley Dowden (eds.), '' Internet Encyclopedia of Philosophy''Eprint

Formal Predicate Calculus

contains a systematic formal development along the lines of Alternative calculus *

forall x: an introduction to formal logic

', by P.D. Magnus, covers formal semantics and

proof theory
Proof theory is a major branchAccording to Wang (1981), pp. 3–4, proof theory is one of four domains mathematical logic, together with model theory, axiomatic set theory, and recursion theory. Barwise (1978) consists of four corresponding parts ...

for sentential logic.Chapter 2 / Propositional Logic

fro

Logic In Action

Propositional sequent calculus prover

on Project Nayuki. (''note'': implication can be input in the form

`!X, Y`

, and a sequent can be a single formula prefixed with `>`

and having no commas)Propositional Logic - A Generative Grammar

{{Authority control Systems of formal logic Logical calculi Boolean algebra Classical logic Analytic philosophy