In

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 s ...

, a universal quantification is a type of quantifier, a logical constant In logic, a logical constant of a language \mathcal is a symbol that has the same semantic value under every interpretation of \mathcal. Two important types of logical constants are logical connectives and quantifiers. The equality predicate ...

which is interpreted as "given any" or "for all". It expresses that a predicate can be satisfied by every member of a domain of discourse
In the formal sciences, the domain of discourse, also called the universe of discourse, universal set, or simply universe, is the set of entities over which certain variables of interest in some formal treatment may range.
Overview
The domai ...

. In other words, it is the predication of a property
Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of the property, an owner of property may have the right to consume, alter, share, r ...

or relation to every member of the domain. It asserts that a predicate within the scope of a universal quantifier is true of every value of a predicate variable In mathematical logic, a predicate variable is a predicate letter which functions as a "placeholder" for a relation (between terms), but which has not been specifically assigned any particular relation (or meaning). Common symbols for denoting predi ...

.
It is usually denoted by the turned A (∀) logical operator
In Mathematical logic, 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 (logic), syntax o ...

symbol
A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different co ...

, which, when used together with a predicate variable, is called a universal quantifier ("", "", or sometimes by "" alone). Universal quantification is distinct from ''existential'' quantification ("there exists"), which only asserts that the property or relation holds for at least one member of the domain.
Quantification in general is covered in the article on quantification (logic)
In logic, a quantifier is an operator that specifies how many individuals in the domain of discourse satisfy an open formula. For instance, the universal quantifier \forall in the first order formula \forall x P(x) expresses that everythi ...

. The universal quantifier is encoded as in Unicode
Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, whi ...

, and as `\forall`

in LaTeX
Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latexes are found in nature, but synthetic latexes are common as well.
In nature, latex is found as a milky fluid found in 10% of all flowering plants (angiosperm ...

and related formula editors.
Basics

Suppose it is given that2·0 = 0 + 0, and 2·1 = 1 + 1, and , etc.This would seem to be a

logical conjunction
In logic, mathematics and linguistics, And (\wedge) is the truth-functional operator of logical conjunction; the ''and'' of a set of operands is true if and only if ''all'' of its operands are true. The logical connective that represents this ...

because of the repeated use of "and". However, the "etc." cannot be interpreted as a conjunction in formal logic
Logic is the study of correct reasoning. It includes both Mathematical logic, formal and informal logic. Formal logic is the science of Validity (logic), deductively valid inferences or of logical truths. It is a formal science investigating h ...

. Instead, the statement must be rephrased:
For all natural numbers ''n'', one has 2·''n'' = ''n'' + ''n''.This is a single statement using universal quantification. This statement can be said to be more precise than the original one. While the "etc." informally includes

natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called ''cardinal ...

s, and nothing more, this was not rigorously given. In the universal quantification, on the other hand, the natural numbers are mentioned explicitly.
This particular example is true
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
* ...

, because any natural number could be substituted for ''n'' and the statement "2·''n'' = ''n'' + ''n''" would be true. In contrast,
For all natural numbers ''n'', one has 2·''n'' > 2 + ''n''is false, because if ''n'' is substituted with, for instance, 1, the statement "2·1 > 2 + 1" is false. It is immaterial that "2·''n'' > 2 + ''n''" is true for ''most'' natural numbers ''n'': even the existence of a single

counterexample
A counterexample is any exception to a generalization. In logic a counterexample disproves the generalization, and does so rigorously in the fields of mathematics and philosophy. For example, the fact that "John Smith is not a lazy student" is a ...

is enough to prove the universal quantification false.
On the other hand,
for all composite number
A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself. Every positive integer is composite, prime, ...

s ''n'', one has 2·''n'' > 2 + ''n''
is true, because none of the counterexamples are composite numbers. This indicates the importance of the ''domain of discourse
In the formal sciences, the domain of discourse, also called the universe of discourse, universal set, or simply universe, is the set of entities over which certain variables of interest in some formal treatment may range.
Overview
The domai ...

'', which specifies which values ''n'' can take.Further information on using domains of discourse with quantified statements can be found in the Quantification (logic)
In logic, a quantifier is an operator that specifies how many individuals in the domain of discourse satisfy an open formula. For instance, the universal quantifier \forall in the first order formula \forall x P(x) expresses that everythi ...

article. In particular, note that if the domain of discourse is restricted to consist only of those objects that satisfy a certain predicate, then for universal quantification this requires a logical conditional
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 premises ...

. For example,
For all composite numbers ''n'', one has 2·''n'' > 2 + ''n''is

logically equivalent
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 premises ...

to
For all natural numbers ''n'', if ''n'' is composite, then 2·''n'' > 2 + ''n''.Here the "if ... then" construction indicates the logical conditional.

Notation

In symbolic logic, the universal quantifier symbol $\backslash forall$ (a turned " A" in asans-serif
In typography and lettering, a sans-serif, sans serif, gothic, or simply sans letterform is one that does not have extending features called "serifs" at the end of strokes. Sans-serif typefaces tend to have less stroke width variation than ser ...

font, Unicode U+2200) is used to indicate universal quantification. It was first used in this way by Gerhard Gentzen
Gerhard Karl Erich Gentzen (24 November 1909 – 4 August 1945) was a German mathematician and logician. He made major contributions to the foundations of mathematics, proof theory, especially on natural deduction and sequent calculus. He died o ...

in 1935, by analogy with Giuseppe Peano's $\backslash exists$ (turned E) notation for existential quantification
In predicate logic, an existential quantification is a type of quantifier, a logical constant which is interpreted as "there exists", "there is at least one", or "for some". It is usually denoted by the logical operator symbol ∃, which, whe ...

and the later use of Peano's notation by 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, ...

.
For example, if ''P''(''n'') is the predicate "2·''n'' > 2 + ''n''" and N is the set of natural numbers, then
: $\backslash forall\; n\backslash !\backslash in\backslash !\backslash mathbb\backslash ;\; P(n)$
is the (false) statement
:"for all natural numbers ''n'', one has 2·''n'' > 2 + ''n''".
Similarly, if ''Q''(''n'') is the predicate "''n'' is composite", then
: $\backslash forall\; n\backslash !\backslash in\backslash !\backslash mathbb\backslash ;\; \backslash bigl(\; Q(n)\; \backslash rightarrow\; P(n)\; \backslash bigr)$
is the (true) statement
:"for all natural numbers ''n'', if ''n'' is composite, then ".
Several variations in the notation for quantification (which apply to all forms) can be found in the '' Quantifier'' article.
Properties

Negation

The negation of a universally quantified function is obtained by changing the universal quantifier into anexistential quantifier
In predicate logic, an existential quantification is a type of quantifier, a logical constant which is interpreted as "there exists", "there is at least one", or "for some". It is usually denoted by the logical operator symbol ∃, which, whe ...

and negating the quantified formula. That is,
:$\backslash lnot\; \backslash forall\; x\backslash ;\; P(x)\backslash quad\backslash text\; \backslash quad\; \backslash exists\; x\backslash ;\backslash lnot\; P(x)$
where $\backslash lnot$ denotes 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 false ...

.
For example, if is the propositional function " is married", then, for the set of all living human beings, the universal quantification
Given any living person , that person is marriedis written :$\backslash forall\; x\; \backslash in\; X\backslash ,\; P(x)$ This statement is false. Truthfully, it is stated that

It is not the case that, given any living person , that person is marriedor, symbolically: :$\backslash lnot\backslash \; \backslash forall\; x\; \backslash in\; X\backslash ,\; P(x)$. If the function is not true for ''every'' element of , then there must be at least one element for which the statement is false. That is, the negation of $\backslash forall\; x\; \backslash in\; X\backslash ,\; P(x)$ is logically equivalent to "There exists a living person who is not married", or: :$\backslash exists\; x\; \backslash in\; X\backslash ,\; \backslash lnot\; P(x)$ It is erroneous to confuse "all persons are not married" (i.e. "there exists no person who is married") with "not all persons are married" (i.e. "there exists a person who is not married"): :$\backslash lnot\backslash \; \backslash exists\; x\; \backslash in\; X\backslash ,\; P(x)\; \backslash equiv\backslash \; \backslash forall\; x\; \backslash in\; X\backslash ,\; \backslash lnot\; P(x)\; \backslash not\backslash equiv\backslash \; \backslash lnot\backslash \; \backslash forall\; x\backslash in\; X\backslash ,\; P(x)\; \backslash equiv\backslash \; \backslash exists\; x\; \backslash in\; X\backslash ,\; \backslash lnot\; P(x)$

Other connectives

The universal (and existential) quantifier moves unchanged across thelogical 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 co ...

s ∧
Wedge (∧) is a symbol that looks similar to an in-line caret (^). It is used to represent various operations. In Unicode, the symbol is encoded and by \wedge and \land in TeX. The opposite symbol (∨) is called a vel, or sometimes a (desce ...

, ∨, →, and ↚, as long as the other operand is not affected; that is:
:$\backslash begin\; P(x)\; \backslash land\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash land\; Q(y))\; \backslash \backslash \; P(x)\; \backslash lor\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash lor\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; P(x)\; \backslash to\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash to\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; P(x)\; \backslash nleftarrow\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash nleftarrow\; Q(y))\; \backslash \backslash \; P(x)\; \backslash land\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash land\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; P(x)\; \backslash lor\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash lor\; Q(y))\; \backslash \backslash \; P(x)\; \backslash to\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash to\; Q(y))\; \backslash \backslash \; P(x)\; \backslash nleftarrow\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash nleftarrow\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash end$
Conversely, for the logical connectives ↑, ↓, ↛, and ←, the quantifiers flip:
:$\backslash begin\; P(x)\; \backslash uparrow\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash uparrow\; Q(y))\; \backslash \backslash \; P(x)\; \backslash downarrow\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash downarrow\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; P(x)\; \backslash nrightarrow\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash nrightarrow\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; P(x)\; \backslash gets\; (\backslash exists\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash forall\backslash mathbf\backslash ,\; (P(x)\; \backslash gets\; Q(y))\; \backslash \backslash \; P(x)\; \backslash uparrow\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash uparrow\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; P(x)\; \backslash downarrow\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash downarrow\; Q(y))\; \backslash \backslash \; P(x)\; \backslash nrightarrow\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash nrightarrow\; Q(y))\; \backslash \backslash \; P(x)\; \backslash gets\; (\backslash forall\backslash mathbf\backslash ,\; Q(y))\; \&\; \backslash equiv\backslash \; \backslash exists\backslash mathbf\backslash ,\; (P(x)\; \backslash gets\; Q(y)),\&\; \backslash text\; \backslash mathbf\backslash neq\; \backslash emptyset\; \backslash \backslash \; \backslash end$
Rules of inference

A rule of inference is a rule justifying a logical step from hypothesis to conclusion. There are several rules of inference which utilize the universal quantifier. '' Universal instantiation'' concludes that, if the propositional function is known to be universally true, then it must be true for any arbitrary element of the universe of discourse. Symbolically, this is represented as :$\backslash forall\backslash mathbf\backslash ,\; P(x)\; \backslash to\; P(c)$ where ''c'' is a completely arbitrary element of the universe of discourse. '' Universal generalization'' concludes the propositional function must be universally true if it is true for any arbitrary element of the universe of discourse. Symbolically, for an arbitrary ''c'', :$P(c)\; \backslash to\backslash \; \backslash forall\backslash mathbf\backslash ,\; P(x).$ The element ''c'' must be completely arbitrary; else, the logic does not follow: if ''c'' is not arbitrary, and is instead a specific element of the universe of discourse, then P(''c'') only implies an existential quantification of the propositional function.The empty set

By convention, the formula $\backslash forall\backslash emptyset\; \backslash ,\; P(x)$ is always true, regardless of the formula ''P''(''x''); seevacuous truth
In mathematics and logic, a vacuous truth is a conditional or universal statement (a universal statement that can be converted to a conditional statement) that is true because the antecedent cannot be satisfied. For example, the statement "sh ...

.
Universal closure

The universal closure of a formula φ is the formula with nofree variable
In mathematics, and in other disciplines involving formal languages, including mathematical logic and computer science, a free variable is a notation (symbol) that specifies places in an expression where substitution may take place and is ...

s obtained by adding a universal quantifier for every free variable in φ. For example, the universal closure of
:$P(y)\; \backslash land\; \backslash exists\; x\; Q(x,z)$
is
:$\backslash forall\; y\; \backslash forall\; z\; (\; P(y)\; \backslash land\; \backslash exists\; x\; Q(x,z))$.
As adjoint

Incategory theory
Category theory is a general theory of mathematical structures and their relations that was introduced by Samuel Eilenberg and Saunders Mac Lane in the middle of the 20th century in their foundational work on algebraic topology. Nowadays, cat ...

and the theory of elementary topoi, the universal quantifier can be understood as the right adjoint
In mathematics, specifically category theory, adjunction is a relationship that two functors may exhibit, intuitively corresponding to a weak form of equivalence between two related categories. Two functors that stand in this relationship are kno ...

of a functor
In mathematics, specifically category theory, a functor is a mapping between categories. Functors were first considered in algebraic topology, where algebraic objects (such as the fundamental group) are associated to topological spaces, and ma ...

between power set
In mathematics, the power set (or powerset) of a set is the set of all subsets of , including the empty set and itself. In axiomatic set theory (as developed, for example, in the ZFC axioms), the existence of the power set of any set is post ...

s, the inverse image
In mathematics, the image of a function is the set of all output values it may produce.
More generally, evaluating a given function f at each element of a given subset A of its domain produces a set, called the "image of A under (or through) ...

functor of a function between sets; likewise, the existential quantifier
In predicate logic, an existential quantification is a type of quantifier, a logical constant which is interpreted as "there exists", "there is at least one", or "for some". It is usually denoted by the logical operator symbol ∃, which, whe ...

is the left adjoint
In mathematics, specifically category theory, adjunction is a relationship that two functors may exhibit, intuitively corresponding to a weak form of equivalence between two related categories. Two functors that stand in this relationship are kno ...

.Saunders Mac Lane
Saunders Mac Lane (4 August 1909 – 14 April 2005) was an American mathematician who co-founded category theory with Samuel Eilenberg.
Early life and education
Mac Lane was born in Norwich, Connecticut, near where his family lived in Taftvill ...

, Ieke Moerdijk, (1992) ''Sheaves in Geometry and Logic'' Springer-Verlag. ''See page 58''
For a set $X$, let $\backslash mathcalX$ denote its powerset
In mathematics, the power set (or powerset) of a set is the set of all subsets of , including the empty set and itself. In axiomatic set theory (as developed, for example, in the ZFC axioms), the existence of the power set of any set is post ...

. For any function $f:X\backslash to\; Y$ between sets $X$ and $Y$, there is an inverse image
In mathematics, the image of a function is the set of all output values it may produce.
More generally, evaluating a given function f at each element of a given subset A of its domain produces a set, called the "image of A under (or through) ...

functor $f^*:\backslash mathcalY\backslash to\; \backslash mathcalX$ between powersets, that takes subsets of the codomain of ''f'' back to subsets of its domain. The left adjoint of this functor is the existential quantifier $\backslash exists\_f$ and the right adjoint is the universal quantifier $\backslash forall\_f$.
That is, $\backslash exists\_f\backslash colon\; \backslash mathcalX\backslash to\; \backslash mathcalY$ is a functor that, for each subset $S\; \backslash subset\; X$, gives the subset $\backslash exists\_f\; S\; \backslash subset\; Y$ given by
:$\backslash exists\_f\; S\; =\backslash ,$
those $y$ in the image of $S$ under $f$. Similarly, the universal quantifier $\backslash forall\_f\backslash colon\; \backslash mathcalX\backslash to\; \backslash mathcalY$ is a functor that, for each subset $S\; \backslash subset\; X$, gives the subset $\backslash forall\_f\; S\; \backslash subset\; Y$ given by
:$\backslash forall\_f\; S\; =\backslash ,$
those $y$ whose preimage under $f$ is contained in $S$.
The more familiar form of the quantifiers as used in 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 quantifi ...

is obtained by taking the function ''f'' to be the unique function $!:X\; \backslash to\; 1$ so that $\backslash mathcal(1)\; =\; \backslash $ is the two-element set holding the values true and false, a subset ''S'' is that subset for which the predicate $S(x)$ holds, and
:$\backslash begin\backslash mathcal(!)\backslash colon\; \backslash mathcal(1)\; \&\; \backslash to\; \backslash mathcal(X)\backslash \backslash \; T\; \&\backslash mapsto\; X\; \backslash \backslash \; F\; \&\backslash mapsto\; \backslash \backslash end$
:$\backslash exists\_!\; S\; =\; \backslash exists\; x.\; S(x),$
which is true if $S$ is not empty, and
:$\backslash forall\_!\; S\; =\; \backslash forall\; x.\; S(x),$
which is false if S is not X.
The universal and existential quantifiers given above generalize to the presheaf category
In category theory, a branch of mathematics, a presheaf on a category C is a functor F\colon C^\mathrm\to\mathbf. If C is the poset of open sets in a topological space, interpreted as a category, then one recovers the usual notion of presheaf on a ...

.
See also

*Existential quantification
In predicate logic, an existential quantification is a type of quantifier, a logical constant which is interpreted as "there exists", "there is at least one", or "for some". It is usually denoted by the logical operator symbol ∃, which, whe ...

* 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 quantifi ...

* 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 sub ...

—for the Unicode symbol ∀
Notes

References

* * (ch. 2)External links

* {{Mathematical logic Logic symbols Logical expressions Quantifier (logic)