[FOM] n-th order ZFC
William Tait
williamtait at mac.com
Sat Jul 9 12:29:26 EDT 2011
On Jul 6, 2011, at 10:21 PM, W.Taylor at math.canterbury.ac.nz wrote:
> I have read long ago that in some sense 3rd-order (or higher) is unnecessary,
> in that 3rd-order can be somehow "mirrored" in second-order, with no loss
> of fidelity. Or some such comment.
In second-order logic with the epsilon relation, let T have the axioms
1. there is a (von Neumann) ordinal alpha such that all ordinals are <= alpha +n,
2. for W=V_{alpha} and phi the conjunction of the axioms of second-erder ZF(C), phi^W (the relativization of phi to W).
[It is possible to do this because second-order ZF is finitely axiomatizable.]
3. the definition of V_{beta} is extended to beta greater or equal to alpha and V_{beta} is postulated to contain, for each class whose elements are of rank < beta, a set with the same elements (i.e. the second-order axiom of Separation is extended to all the ordinals).
In this theory there is an obvious translation of ZF of order n+2.
Best wishes,
Bill Tait
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