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 Maarten Maartensz:    Philosophical Dictionary | Filosofisch Woordenboek                      

 Q - Quantifier

 

Quantifier: Qualitative term that states the proportion of something in some domain.

In logic the standard quantifiers are "for all" or "for every" or "for each" and "for some" or "there is" or "there are", and in standard logic the two are interdefinable, in that "for all x Ax" is true precisely if "it is not true that for some x not Ax" and "for some x Ax" is true precisely if "it is not true that for all x not Ax".

There are many subtle issues involved in the logic of quantifiers, which was first thought of and created by Frege and, independently, Peirce and Mitchell.

Five problems relating to quantifiers are as follows.

First, there is the problem that "there is an x such that Fx" and "for some x, Fx" are not intuitively the same, as shown by "Some Greek Gods dallied with human damsels, though - of course - there are no Greek Gods".

This can be rectified in various ways, e.g. by a predicate of existence, or by laying down that a thing exists iff some thing is identical to it, as in "(Ex)(x=Napoleon) & ~(Ex)(x=Jupiter)". Both remedies have their own problems.

Second, there is the problem of quantifying subjects and predicates. Standard logic and Set Theory has only quantified subjects and not quantified predicates, which is to say it has a First Order Logic.

The problem is that it is quite intuitive in Natural Language to quantify predicates ("You don't have all properties Einstein had") and that quantified predicates make it easier to deal with quite a few logical and mathematical concepts, but that, on the other hand, there are logical problems involved in quantified predicates.

One basic reason for this is that quantified predicates and relations assert far more than constant terms with quantified subjects only, and that this easily leads to paradoxes and problems. Logics with quantified predicates are known as Higher Order Logic a.k.a. HOL.

There are now good introductions to HOL, but it also has unresolved problems.

Third, Henkin showed that not all things can be said precisely and uniquely using formulas with sequences of quantifier prefixes. This is known as the problem of Branching Quantifiers, which is essentially due to dependencies between universally and particularly quantified terms.

To accomodate such quantifiers, some things have to be adjusted or assumed.

Fourth, there are other quantifiers than "for all x", "for some x" and "there is an x such that", such as "for most x" and "for a few x", and also quantifying expressions in temporal logic, as "for all times", "for some times" etc.

Fifth, there are conceptions of mathematics and logic (intuitionism and constructivism) that insist that "there is an x such that Fx" is true and sensible only if one can show how to construe something that is F, and that one cannot merely hypostasize this or conclude if from a reductio ad absurdum.

The problem with this is that most mathematicians have liked constructive approaches, but found them insufficient for some things they wanted to express, and that at least sometimes such non-constructive statements seem both sensible and true, as in "there are particular numbers no one has ever thought of, specifically".

Even so, it has been shown, e.g. by Errett Bishop, that far more can be said and proved constructively than most mathematicians would have guessed - and the advantage of constructive proofs is that involve fewer assumptions than non-constructive proofs.

In general terms:

The standard quantifiers have been most researched and studied, mostly by mathematicians and logicians, but more recently also by linguists, computer scientists and even philosophers, and there are, next to the standard approach to quantification, systems for alternative quantifiers, and temporal logics with temporal quantifiers, and there also are different approaches to quantifiers, notably in intuitionist and constructive mathematics.

And in any case, the logical notion of a quantifier is an example of a very neat and useful abstraction thought up by logical mathematicians in the 19th Century, of which the clarification of their logic - the rules and assumptions involved in their logical use - was most useful for both mathematics and logic.

 


See also: Every, First Order Logic, Higher Order Logic, Some, Logical terms


Literatuur:

Carnap, Cartwright, Eyck & Thysse, Frege

 Original: Oct 14, 2004                                                Last edited: 12 December 2011.   Top