Philosophy Lexicon of Arguments

One, number 1: in modern logic it is not possible to introduce the number one directly. It must be introduced indirectly, via existential quantification ("for at least one x ...") and universal quantification ("for all x ..."). In addition, identity is needed. See also definition, identity, logic, elementary logic, number theory, numbers.
Author Item Excerpt Meta data
Hilbert, D.
Books on Amazon
One (Number 1) Berka I 121
Definition 1/One/Number/Logical form/Hilbert:

1(F) : (Ex)[F(x) & (y)(F(y) > ≡ (x,y)].

Hilbert: "There is an x for which F(x) exists, and every y for which F(y) exists is identical with this x".

Definition 2/two/number/logical form/Hilbert:

2(F) :(Ex)(Ey) {~≡(x,y) & F(x) & F(y) & (z)[F(z) > ≡ (x,z) v ≡ (y,z)]}.

I 122
"There are two different x and y to which F applies, and every z for which F(z) exists is identical with x or y".

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

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Ed. Martin Schulz, access date 2017-04-24