In mathematical logic, formation rules are rules for describing which strings of symbols formed from the

alphabet An alphabet is a standardized set of basic written graphemes (called letters) that represent the phonemes of certain spoken languages. Not all writing systems represent language in this way; in a syllabary, each character represents a syllab ...

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 sy ...

are syntactically valid within the language. These rules only address the location and manipulation of the strings of the language. It does not describe anything else about a language, such as its

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 comp ...

(i.e. what the strings mean). (See also

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 ...

Formal language

A ''formal language'' is an organized set of symbols the essential feature being that it can be precisely defined in terms of just the shapes and locations of those symbols. Such a language can be defined, then, without any reference to any meanings of any of its expressions; it can exist before any interpretation is assigned to it—that is, before it has any meaning. A

determines which symbols and sets of symbols are formulas in a formal language.

Formal systems

A ''formal system'' (also called a ''logical calculus'', or a ''logical system'') consists of a formal language together with a

deductive apparatus 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 form ...

(also called a ''deductive system''). The deductive apparatus may consist of a set of transformation rules (also called ''inference rules'') or a set of axioms, or have both. A formal system is used to derive one expression from one or more other expressions. Propositional and predicate calculi are examples of formal systems.

Propositional and predicate logic

The formation rules of a

propositional calculus Propositional calculus is a branch of logic. It is also called propositional logic, statement logic, sentential calculus, sentential logic, or sometimes zeroth-order logic. It deals with propositions (which can be true or false) and relations ...

may, for instance, take a form such that; * if we take Φ to be a propositional formula we can also take Φ to be a formula; * if we take Φ and Ψ to be a propositional formulas we can also take (Φ Ψ), (Φ Ψ), (Φ Ψ) and (Φ Ψ) to also be formulas. A

predicate calculus Predicate or predication may refer to: * Predicate (grammar), in linguistics * Predication (philosophy) * several closely related uses in mathematics and formal logic: **Predicate (mathematical logic) **Propositional function ** Finitary relation, ...

will usually include all the same rules as a propositional calculus, with the addition of quantifiers such that if we take Φ to be a formula of propositional logic and α as a variable then we can take (α)Φ and (α)Φ each to be formulas of our predicate calculus.

References

{{Mathematical logic Formal languages Propositional calculus Predicate logic Rules Syntax (logic) Logical truth