In Introduction to Metamathematics, Kleene introduces a formal system where the first three postulat

therightwomanwf

therightwomanwf

Answered question

2022-07-01

In Introduction to Metamathematics, Kleene introduces a formal system where the first three postulates in the group for propositional calculus are:
1 a . A ( B A ) 1 b . ( A B ) ( ( A ( B C ) ) ( A C ) ) 2. A , A B B
As far as I understand, 1 a and 2 are typical for Hilbert-style deductive systems, but 1 b is not. A more traditional choice, serving pretty much the same purpose (e.g. proving A A to start with) would have been:
( A ( B C ) ) ( ( A B ) ( A C ) )
What is the rationale for the unique choice made for 1 b in Introduction to Metamathematics?

Answer & Explanation

jugf5

jugf5

Beginner2022-07-02Added 18 answers

Let's write:
( 1 b ) ( A B ) ( ( A ( B C ) ) ( A C ) ) ( 1 b ) ( A ( B C ) ) ( ( A B ) ( A C ) )
Intuitively, ( 1 b ) and ( 1 b ) have the same meaning: If you know ( A B ) and you know ( A ( B C ) ), then you can conclude ( A C ), by using Modus Ponens (2) twice. The order in which the hypotheses are stated doesn't matter.

The role of ( 1 b ) / ( 1 b ) in a Hilbert system is to "internalize" Modus Ponens in order to prove the deduction theorem. Both ( 1 b ) and ( 1 b ) are adequate to prove the deduction theorem, and once we have the deduction theorem in place, our particular choice of axioms for doesn't matter.

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