Some General Remarks on Provability and Truth

Abstract

We have given three different incompleteness proofs of Peano Arithmetic— the first used Tarski’s truth-set, the second (Gödel’s original proof) was based on the assumption of ω-consistency, and the third (Rosser’s proof) was based on the assumption of simple consistency. The three proofs yield different generalizations—namely 1. Every axiomatizable subsystem of N is incomplete. 2. Every axiomatizable ω-consistent system in which all true Σ0-sentences are provable is incomplete. 3. Every axiomatizable simply consistent extension of (R) is incomplete. The first of the three proofs is by far the simplest and we are surprised that it has not appeared in more textbooks. Of course, it can be criticized on the grounds that it is not formalizable in arithmetic (since the truth set is not expressible in arithmetic), but this should be taken with some reservations in light of Askanas’ theorem, which we will discuss a bit later. It is not too surprising that Peano Arithmetic is incomplete because the scheme of mathematical induction does not really express the full force of mathematical induction. The true principle of mathematical induction is that for any set A of natural numbers, if A contains 0 and A is closed under the successor function (such a set A is sometimes called an inductive set), then A contains all natural numbers. Now, there are non-denumerably many sets of natural numbers but only denumerably many formulas in the language LA and, hence, there are only denumerably many expressible sets of LA- Therefore, the formal axiom scheme of induction for P.A. guarantees only that for every expressible set A, if A is inductive, then A contains all natural numbers. To express the principle of mathematical induction fully, we need second order arithmetic in which we take set and relational variables and quantify over sets and relations of natural numbers.