@pam69ur,
Yes, but you selectively leave out the whole premise of proposition VI:
Quote:If, instead of w-consistency, mere consistency as such is assumed for
c, then there follows, indeed, not the existence of an undecidable proposition, but rather the existence of a property
(r) for which it is possible neither to provide a counter-example nor to prove that it holds for all numbers. For, in proving that
17 Gen r is not
c-provable, only the consistency of
c is employed (cf. [189]) and from
~Bewc(17 Gen r) it follows, according to (
15), that for every number
x,
Sb(r 17|z(x)) is
c-provable, and hence that
Sb(r 17|Z(x)) is not
c-provable for any number.
By adding
Neg(17 Gen r) to
c, we obtain a consistent but not w-consistent class of
formulae c'.
c' is consistent, since otherwise
17 Gen r would be
c-provable.
c' is not however w-consistent, since in virtue of
~Bewc(17 Gen r) and (
15) we have:
(x) BewcSb(r 17|Z(x)), and so
a fortiori:
(x) Bewc'Sb(r 17|Z(x)), and on the other hand, naturally:
Bewc'[Neg(17 Gen r)].
46
A special case of
Proposition VI is that in which the class
c consists of a finite number of
formulae (with or without those derived therefrom by
type-lift). Every finite class
a is naturally recursive. Let
a be the largest number contained in
a. Then in this case the following holds for
c:
[CENTER]
[/CENTER]
c is therefore recursive.
This allows one, for example, to conclude that even with the help of the axiom of choice (for all types), or the generalized continuum hypothesis, not all propositions are decidable, it being assumed that these hypotheses are w-consistent.
And immediately after the part you quote:
Quote:
Hence in every formal system that satisfies assumptions 1 and 2 and is w-consistent, undecidable propositions exist of the form (x) F(x), where F is a recursively defined property of natural numbers, and so too in every extension of such a system made by adding a recursively definable w-consistent class of axioms. As can be easily confirmed, the systems which satisfy assumptions 1 and 2 include the Zermelo-Fraenkel and the v. Neumann axiom systems of set theory,
47 and also the axiom system of number theory which consists of the Peano axioms, the operation of recursive definition [according to schema (2)] and the logical rules.
48 Assumption 1 is in general satisfied by every system whose rules of inference are the usual ones and whose axioms (like those of P) are derived by substitution from a finite number of schemata.
48a
and in proposition VI itself he also says this:
Quote:Every w-consistent system is naturally also consistent. The converse, however, is not the case, as will be shown later.
The general result as to the existence of undecidable propositions reads:
Proposition VI: To every w-consistent recursive class c of formulae there correspond recursive class-signs r, such that neither v Gen r nor Neg (v Gen r) belongs to Flg(c) (where v is the free variable of r).
He gives the proof, then concludes (I've truncated this):
Quote:1.
17 Gen r is not
c-provable
2.
Neg(17 Gen r) is not
c-provable.
Neg(17 Gen r) is therefore undecidable in
c, so that
Proposition VI is proved.
Godel is
CLEARLY constructing his proof around the ramifications of a certain kind of axiomatic statement as well as the converse of that statement. The validity of that axiom is irrelevant, because it can only be valid in relation to the theorem that contains it. However, its
structure is what is relevant, and that's the whole basis of this (and every other) proposition.