[FOM] Discussion between Thomas Forster & Bob Solovay
Martin Davis
martin at eipye.com
Mon Sep 9 14:44:32 EDT 2002
Thomas Forster wrote
>My understanding is that if you set up a weak set theory
>which is nevertheless strong enough to prove that there are
>infinitely many hereditarily finite sets as a natural deduction system, then
>the obvious proof turns out to have a maximal formula.
> My point is simply the general one that any proof in
>a consistent set theory that involves sanitizing a paradox
>is likely to be pathological.
Let HF be the set of hereditarily finite sets. Let V_n be defined as usual:
V_0 = emptyset; V_{n+1}= Powerset[V_n]. Then HF is the union of the V_n and
and |V_n|=2^n. Hence there is no natural number n such that |HF|=n.
Does this proof "involve sanitizing a paradox"
Alternative proof (using a different definition of infinite set): Choosing
one of the obvious coding of hereditarily finite sets by natural numbers,
we have a one-one correspondence between HF and N, and therefore (using the
von Neumann coding of natural numbers by sets) with a proper subset of itself.
Same question. Really anything odd about the formalized version of these
proofs in a natural deduction system (with appropriate set theortic axioms)?
Martin
Martin Davis
Visiting Scholar UC Berkeley
Professor Emeritus, NYU
martin at eipye.com
(Add 1 and get 0)
http://www.eipye.com
More information about the FOM
mailing list