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Universal and Existential Quantifiers: Semantics and Syntax, Study notes of Philosophy

University of California - DavisPhilosophy

An introduction to universal and existential quantifiers in philosophy and logic. It covers the syntax and semantics of universal sentences, the behavior of universal quantifiers, the concept of free variables and open sentences, and the interpretation of universally quantified sentences. The document also introduces existential quantifiers, their behavior, and the interpretation of existentially quantified sentences. Examples and cross-references to predicate logic.

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Introduction to Quantifiers
G. J. Mattey
Winter, 2009 / Philosophy 112
Universal Sentences
•Many sentences of natural language make assertions about whole classes of in-
dividuals.
•Some of these sentences were called by Aristotle universal sentences, though
we will call them all “universal.”
–Everyone loves Adam.
•Universal sentences begin with a quantity term (‘all,’ ‘every,’ ‘any,’ ‘everybody,’
etc.) which may only be implicit, as in the following example.
–Horses are mammals.
•We would like to be able to symbolize universal sentences, because they play an
important role in inference.
–1. Everyone loves Adam.
2. Therefore, Eve loves Adam.
The Syntax of Universal Sentences
•Many universal sentences have a quantity term in the subject position of the
sentence.
–Everyone loves Adam.
•Other universal sentences have quantity term modifying a general term in the
subject position of the sentence.
–All horses are mammals.
•Still other universal sentences do not display the quantity term at all.
–Horses are mammals.
–A horse is a mammal.
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Introduction to Quantifiers

G. J. Mattey

Winter, 2009 / Philosophy 112

Universal Sentences

  • Many sentences of natural language make assertions about whole classes of in- dividuals.
  • Some of these sentences were called by Aristotle universal sentences, though we will call them all “universal.” - Everyone loves Adam.
  • Universal sentences begin with a quantity term (‘all,’ ‘every,’ ‘any,’ ‘everybody,’ etc.) which may only be implicit, as in the following example. - Horses are mammals.
  • We would like to be able to symbolize universal sentences, because they play an important role in inference. - 1. Everyone loves Adam. 2. Therefore, Eve loves Adam.

The Syntax of Universal Sentences

  • Many universal sentences have a quantity term in the subject position of the sentence. - Everyone loves Adam.
  • Other universal sentences have quantity term modifying a general term in the subject position of the sentence. - All horses are mammals.
  • Still other universal sentences do not display the quantity term at all.
    • Horses are mammals.
    • A horse is a mammal.

The Semantics of Universal Sentences

  • Semantically, universal quantity terms do not play the role either of subjects or of predicates. - They do not designate a single individual, as does a subject of a sentence. - They do not say anything about individuals or sets of individuals, as does the predicate of a sentence.
  • Instead, universal quantity terms designate the class of all individuals.
  • The sentence to which they apply says something about all the members of that class.

Displaying the Behavior of Universal Sentences

  • The semantical behavior of the quantity term in the subject position is best brought out in the following formulation.
  • Everything is such that it [satisfies the condition stated by the rest of the sen- tence]. - Everything is such that it loves Adam.
  • The semantical behavior of the quantity term modifying a general term in the subject position can be formulated this way.
  • Everything is such that if it [falls under the general term], then it [satisfies the condition stated by the rest of the sentence]. - Everything is such that if it is a horse, then it is a mammal.

The Universal Quantifier

  • In Predicate Logic, the role of ‘every’ in ‘everything’ is played by the universal symbol, ‘∀.’
  • The role of ‘thing’ in ‘everything’ is played by a variable, ‘w,’ ‘x,’ ‘y,’ ‘z’ (with or without positive integer subscripts).
  • The whole expression ‘everything is such that’ combines the universal symbol with a variable, as in ‘(∀x).’
  • This expression of Predicate Logic is called the universal quantifier.
  • The quantifier binds all the occurrences in the sentence it governs of the variable it contains. - In the sentence ‘(∀x)(Hx ⊃ Mx),’ ‘(∀x)’ binds both occurrences of ‘x’ in ‘Hx ⊃ Mx.’ - In the sentence ‘(∀x)Hx ⊃ Mx,’ ‘(∀x)’ binds the occurrence of ‘x’ in ‘Hx.’

Free Variables and Open Sentences

  • A variable is free in a sentence when it is not bound by any quantifier in that sentence. - In the sentence ‘(∀x)(Hx ⊃ My),’ ‘x’ is bound and ‘y’ is free.
  • A sentence of Predicate Logic which contains at least one free variable is an open sentence.
  • Some logicians do not consider “open sentences” to be sentences, because they contain terms (variables) which have no intended reference. - The sentence ‘(∀x)(Hx ⊃ My)’ would be transcribed into quasi-English as: Everything x is such that if x is a horse, then y is a mammal.
  • We count open sentences as sentences for simplicity.

Vacuous Quantification

  • The universal quantifier is an operator that creates a sentence of Predicate Logic when prefixed to a sentence of Predicate Logic.
  • Sometimes prefixing a universal quantifier to a sentence does not bind a variable.
    • (∀y)(Ha ⊃ Mb)
  • Such cases are called cases of vacuous quantification.
  • We will treat vacuous quantifiers semantically as if they were not there at all.

Interpreting Universally Quantified Sentences

  • The ‘everything’ intended to be captured by the universal quantifier is reflected in the domain of an interpretation.
  • If in an interpretation the domain consists of two people, Adam and Eve, then they are ‘everything’ according to that interpretation.
  • So for a univerally quantified sentence (∀u)P(u) to be true, it is required that every object in the domain meet the condition specified by the open sentence P(u).
  • ‘(∀x)Lxa’ is true just in case both Adam and Eve meet the condition speci- fied by ‘Lxa.’
  • v(a) = Adam, requires that both 〈Adam, Adam〉 and 〈Eve, Adam〉 be in v(L) for ‘(∀x)Lxa’ to be true.

L x a ↓ ↓ 〈Adam, Adam〉 and 〈Eve, Adam〉

Substitution Instances

  • Universally quantified sentences that are not components of other sentences can be converted to substitution instances by dropping the quantifier and uniformly substituting a constant term for all the occurrences of the variable in the quanti- fier. - ‘(∀x)(Hx ⊃ Mx)’ −→ ‘Ha ⊃ Ma.’
  • The constant term is called the instantiating constant.
  • More generally, a substitution instance of (∀u)(.. .u.. .) is (.. .s/u.. .), where (.. .s/u.. .) is (.. .u.. .) except that all occurrences of u are replaced with s.
  • Manipulation of substitution instances is the most important kind of move in doing Predicate Logic derivations.

Satisfaction

  • A problem stated in the text is that open sentences have no truth-values.
  • Nonetheless, we would like to say something about what would happen to an open sentence if we were to let its variable stand for a member of the domain.
  • We will say that under this condition, the open sentence is satisfied.
    • If in an interpretation ‘x’ is assumed to stand for Adam and 〈Adam〉 ∈ v(B), then ‘Bx’ is satisfied given that assumption in the interpretation.
  • But as yet we have no means to indicate what variables stand for.

Variable Assignments

  • We will expand our semantics by introducing, as components of interpretations, variable assignments d 1 , d 2 ,... whose arguments are variables and whose val- ues are members of the domain of that interpretation.
  • For example, in an interpretation whose domain is {Adam, Eve}, then d 1 (x) might assign ‘x’ to Adam and d 2 (x) might assign ‘x’ to Eve.

Truth-Definition for Universally Quantified Sentences

  • Let ‘d[u/x]’ indicate a variable assignment just like d with the possible exception of the assignment of a member of the domain u to x. - Suppose d(x) = Adam. - Then dEve/x = Eve.
  • ‘d[u/x]’ is called an x-variant of d.
  • d satisfies a universally quantified sentence (∀x)P(x) in an interpretation I if and only if P(x) is satisfied by the x-variants of d d[u/x] for all u in the domain.
  • A sentence P of Predicate Logic is true in an interpretation I if and only if P is satisfied by all variable assignments, which can be seen if an arbitrary variable assignment d satisfies it.
  • It can be proved that every sentence of predicate logic is satisfied by either all variable assignments or no variable assignments, so every (non-open) sentence of predicate logic is either true or false in a given interpretation.

An Example

  • For an interpretation I, D = {Adam, Eve}, v(L) = {〈Adam, Adam〉, 〈Eve, Adam〉}, v(a) = Adam.
  • d[Adam/x] satisfies ‘Lxa.’
    • 〈dAdam/x, v(a)〉 ∈ v(L).
    • 〈desd,vAdam/x, desd,v(a)〉 ∈ v(L).
  • d[Eve/x] satisfies ‘Lxa.’
    • 〈dEve/x, v(a)〉 ∈ v(L).
    • 〈desd,vEve/x, desd,v(a)〉 ∈ v(L).
  • So, the x-variants of d for all members of the domain satisfy ‘Lxa.’
  • So, d satisfies ‘(∀x)Lxa.’
  • Since the choice of d is arbitrary, all variable assignments satisfy ‘(∀x)Lxa,’ so the sentence is true in I.

Substitutional Semantics for Universally Quantified Sentences

  • We have said that for a universally quantified sentence to be true, all members of the domain must satisfy the condition specified by the sentence following the quantifier.
  • One way to understand the notion of satisifying the condition specified by the sentence following the quantifier is in terms of the truth of substitution instances of the quantified expression. - ‘(∀x)Lxa’ is true if and only if the condition specified by ‘Lxa’ is satisfied by all members of the domain. - Suppose D = {Adam, Eve}, and ‘a’ designates Adam while ‘e’ designates Eve. - Then the sentence is true if and only if ‘Laa’ is true and ‘Lea’ is true. - This is because ‘Laa’ is true if and only if 〈Adam, Adam〉 is in the extension of ‘L,’ and ‘Lea’ is true if and only if 〈Eve, Adam〉 is in the extension of ‘L.’

Particular or Existential Sentences

  • Many sentences of natural language make assertions about at least one, unspeci- fied, individual.
  • Some of these sentences are called by Aristotle particular sentences, though we will call them all “particular.’ - Someone loves Adam.
  • Particular sentences begin with an “existential” quantity term (some, there is a(n), there is at least one, there exists). - Some horses are mares.
  • We would like to be able to symbolize particular sentences, because they play an important role in inference. - Eve loves Adam. Therefore, someone loves Adam.

The Syntax of Particular Sentences

  • Many particular sentences have a quantity term in the subject position of the sentence. - Someone loves Adam.
  • Other particular sentences have quantity term modifying a general term in the subject position of the sentence.

Transcribing Particular Sentences

  • Now we are in a position to display the link between the existential quantifier and the expression containing the variable, first with the quantity term in the subject position. - Someone loves Adam. - Some x is such that x loves Adam. - (∃x)Lxa, where Lxy: x loves y, a: Adam.
  • Now with the quantity term modifying a general term.
    • Some horse is a mare.
    • Some x is such that x is a horse and x is a mare.
    • (∃x)(Hx & Mx), where Hx: x is a horse, and Mx: x is a mare.

Uniform Behavior of Quantifiers

  • Much of the terminology applied to universal quantifiers can be applied to exis- tential quantifiers.
  • An existential quantifier governs the shortest full sentence following it, and it binds occurrences of its variable in the governed sentence.
  • In cases of vacuous quantification, the sentence is interpreted as if the quantifier were not there.
  • A substitution instance of an existentially quantified sentence is the sentence governed by the quantifier with all the occurances of the binding variable being replaced by a constant term.

Interpreting Existentially Quantified Sentences

  • The ‘something’ intended to be captured by the existential quantifier is reflected in the domain of an interpretation.
  • If in an interpretation the domain consists of two people, Adam and Eve, then either one of the two (or both) is the ‘something’ according to that interpretation.
  • So for an existentially quantified sentence (∃u)P(u) to be true, it is required that at least one object in the domain meet the condition specified by the open sentence P(u). - ‘(∃x)Lxa’ is true just in case either Adam or Eve (inclusively) meet the condition specified by ‘Lxa.’ - Given that ‘a’ designates Adam, this means that either 〈Adam, Adam〉 or 〈Eve, Adam〉 is in the extension of ‘L.’

L x a ↓ ↓ 〈Adam, Adam〉 or 〈Eve, Adam〉

Truth-Definition for Existentially Quantified Sentences

  • d satisfies an existentially quantified sentence (∃x)P(x) in an interpretation I if and only if P(x) is satisfied by an x-variant of d d[u/x] for some u in the domain.
  • For an interpretation I, D = {Adam, Eve}, v(L) = {〈Adam, Adam〉, 〈Eve, Adam〉}, v(a) = Adam.
  • d[Eve/x] satisfies ‘Lxa.’
  • So, an x-variant of d for some member of the domain satisfies ‘Lxa.’
  • So, d satisfies ‘(∃x)Lxa.’
  • Since the choice of d is arbitrary, all variable assignments satisfy ‘(∃x)Lxa,’ so the sentence is true in I.

Substitutional Semantics for Existentially Quantified Sentences

  • We have said that for an existentially quantified sentence to be true, at least one member of the domain must satisfy the condition specified by the sentence following the quantifier.
  • One way to understand the notion of satisifying the condition specified by the sentence following the quantifier is in terms of the truth of substitution instances of the quantified expression. - ‘(∃x)Lxa’ is true if and only if the condition specified by ‘Lxa’ is satisfied by at least one member of the domain. - Suppose D = {Adam, Eve}, and ‘a’ designates Adam while ‘e’ designates Eve. - Then the sentence is true if and only if ‘Laa’ is true or ‘Lea’ is true. - This is because ‘Laa’ is true if and only if 〈Adam, Adam〉 is in the extension of ‘L,’ and ‘Lea’ is true if and only if 〈Eve, Adam〉 is in the extension of ‘L.’