Predicate Logic --- Level A

Quantification --- Forming Propositions from Predicates

Subjects to be Learned

universe

universal quantifier

existential quantifier

free variable

bound variable

scope of quantifier

order of quantifiers

Contents

A predicate with variables is not a proposition.  For example, the statement x > 1 with variable x over the universe of real numbers is neither true nor false since we don't know what x is.  It can be true or false depending on the value of x.

For x > 1 to be a proposition either we substitute a specific number for x or change it to something like "There is a number x for which x > 1 holds", or "For every number x, x > 1 holds". 

More generally, a predicate with variables ( atomic formula ) can be made a proposition by applying one of the following two operations to each of its variables:

assign a value to the variable quantify the variable using a quantifier (see below). 

For example, x > 1 becomes 3 > 1 if 3 is assigned to x, and it becomes a true statement, hence a proposition. 

In general, a quantification is performed on formulas of predicate logic (called wff ), such as x > 1 or P(x), by using quantifiers on variables.  Note, however, that the quantification in general produces another wff from a wff.  It does NOT always produce a proposition from a wff (See From wff to Proposition for more on this).  There are two types of quantifiers: universal quantifier and existential quantifier. 

The universal quantifier turns, for example, the statement x > 1 to "for every object x in the universe, x > 1", which is expressed as " x x > 1".  This new statement is true or false in the universe of discourse.  Hence it is a proposition. 

Similarly the existential quantifier turns, for example, the statement x > 1 to "for some object x in the universe, x > 1", which is expressed as " x x > 1."  Again, it is true or false in the universe of discourse, and hence it is a proposition. 







Universe of Discourse

universe

The Universal Quantifier The expression: x P(x), denotes the universal quantification of the atomic formula P(x).  Translated into the English language, the expression is understood as: "For all x, P(x) holds" or "for every x, P(x) holds".  is called the universal quantifier , and x means all the objects x in the universe.  If this is followed by P(x) then the meaning is that P(x) is true for every object x in the universe.  For example, "All cars have wheels" could be transformed into the propositional form, x P(x), where:

P(x) is the predicate denoting: x has wheels , and

is the predicate denoting: , and the universe of discourse is only populated by cars. 

1

2

3

n

The Existential Quantifier The expression: xP(x), denotes the existential quantification of P(x).  Translated into the English language, the expression could also be understood as: "There exists an x such that P(x)" or "There is at least one x such that P(x)" is called the existential quantifier , and x means at least one object x in the universe.  If this is followed by P(x) then the meaning is that P(x) is true for at least one object x of the universe.  For example, "Someone loves you" could be transformed into the propositional form, x P(x), where:

P(x) is the predicate meaning: x loves you ,

is the predicate meaning: , The universe of discourse contains (but is not limited to) all living creatures. 

Existential Quantifier and Connective OR

If all the elements in the universe of discourse can be listed, then the existential quantification xP(x) is equivalent to the disjunction: P(x 1 ) P(x 2 ) P(x 3 ) ... P(x n ). 



For example, in the above example of x P(x), if we knew that there were only 5 living creatures in our universe of discourse (say: me, he, she, rex and fluff), then we could also write the statement as: P(me) P(he) P(she) P(rex) P(fluff)



An appearance of a variable in a wff is said to be bound if either a specific value is assigned to it or it is quantified.  If an appearance of a variable is not bound, it is called free .  The extent of the application(effect) of a quantifier, called the scope of the quantifier, is indicated by square brackets [ ].  If there are no square brackets, then the scope is understood to be the smallest wff following the quantification. 

For example, in x P(x, y), the variable x is bound while y is free.  In x [ y P(x, y) Q(x, y) ] , x and the y in P(x, y) are bound, while y in Q(x, y) is free, because the scope of y is P(x, y).  The scope of x is [ y P(x, y) Q(x, y) ] . 







Order of Application of Quantifiers When more than one variables are quantified in a wff such as y x P( x, y ), they are applied from the inside, that is, the one closest to the atomic formula is applied first.  Thus y x P( x, y ) reads y [ x P( x, y ) ] , and we say for some y, P( x, y ) holds for every x.



The positions of the same type of quantifiers can be switched without affecting the truth value as long as there are no quantifiers of the other type between the ones to be interchanged. 

For example x y z P(x, y , z) is equivalent to y x z P(x, y , z), z y x P(x, y , z), etc.  It is the same for the universal quantifier. 

However, the positions of different types of quantifiers can not be switched. 

For example x y P( x, y ) is not equivalent to y x P( x, y ).  For let P( x, y ) represent x < y for the set of numbers as the universe, for example.  Then x y P( x, y ) reads "for every number x, there is a number y that is greater than x", which is true, while y x P( x, y ) reads "there is a number y that is greater than any number", which is not true. 







Test Your Understanding of Quantification

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