Philosophy Lexicon of Arguments

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Berka I 122
Definition number/logical form/extended function calculus/Hilbert: also the general number concept can be formulated logically: If a predicate-predicate φ (F) should be a number, then φ must satisfy the following conditions:
1. For two equal predicates F and G, φ must be true for both or none of them.
2. If two predicates F and G are not equal in number, φ can only be true for one of the two predicates F and G.
Logical form:

(F)(G){(φ(F) & φ(G) > Glz (F,G) & [φ(F) & Glz (F,G) > φ(G)]}.

The entire expression represents a property of φ. If we designate it with Z (φ), then we can say:

A number is a predicate-predicate φ that has the property Z (φ).

Problem/> Infinity axiom/Hilbert: a problem occurs when we ask for the conditions under which two predicate-predicates φ and ψ define the same number with the properties Z (φ) and Z (ψ).
Infinity axiom/Equal numbers/Hilbert: the condition for equal numbers or for the fact that two predicate-predicates φ and ψ define the same number is that, that φ(P) and ψ(P) are true for the same predicates P and false for the same predicates. So that the relationship arises:

(P)(φ(P) ↔ ψ(P))

I 122
Problem: when the object area is finite, all the numbers are made equal which are higher than the number of objects in the individual area. For example, if a number is e.g. smaller than 10 to the power of 60 and if we take φ and ψ the predicates which define the numbers 10 to the power of 60+1 and 10 high 60 + 1, then both φ and ψ do not apply to any predicate P.
The relation

(P)(φ(P) ↔ ψ(P))

Is thus satisfied for φ and ψ, that is, φ and ψ would represent the same number.
Solution/Hilbert: infinity axiom: one must presuppose the individual domain as infinite. A logical proof of the existence of an infinite totality is, of course, dispensed with.

Brk I
K. Berka/L. Kreiser
Logik Texte Berlin 1983


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Ed. Martin Schulz, access date 2017-05-27