# [FOM] re: What do you lose if you ditch Powerset?

A.P. Hazen a.hazen at philosophy.unimelb.edu.au
Sat Nov 15 02:15:28 EST 2003

```    Here's the OBVIOUS answer to Timothy Chow's question (with further
questions).
Under appropriate codings, the hereditarily finite sets are "the
same thing" as the natural numbers: ZF without INFINITY is (modulo
painful coding and decoding) about the same thing as First Order PA.
(MY question: is there a reference for details on this?  And for
variants: ZFC without INF, Z without INF?  Are these really "the same
thing" when you make that notion precise as, say, mutual
interpretability?  These are things that ought to be well-known, but
I don't know of a published source.)
Much weaker systems suffice to prove the existence of each
individual Hereditarily Finite Set: the Tarski-Szmielew system
axiomatized by Extensionality, Nullset and Addamember ("for all x and
y there is a z having as members  y and all the members of x"), for
instance.  SECOND-ORDER versions of any system in the neighborhood
will be similar in strength to Second Order PA.  Hao Wang studied
some of these systems around 1950: the relevant papers were
incorporated into his "A Survey of Mathematical Logic" (which, to
spread bibliographical joy, was reprinted as "Logic, Computers and
Sets").
INFINITY gets you denumerable sets (and REPLACEMENT then gets you
lots more denumerable sets), so SECOND Order PA.  I believe it is a
well-known "folklore" result that ZF without POWERSET is
proof-theoretically equivalent to 2nd-Order PA ("Analysis")-- is, in
other words "the same thing" if coding and decoding don't bother you.
Steve Simpson's book on PA2 ("Systems of Second Order Arithmetic")
will list interesting things that ARE provable in this.
Chow's question, however, refers to "making sense of" theorems, so
maybe proof-theoretic equivalence isn't all one could ask for.