[FOM] second-order ZF
Robert Black
Mongre at gmx.de
Wed Dec 17 17:45:51 EST 2003
Dear FOMers,
The following question only makes sense if you think (or are willing
to pretend to think) that we have an unambiguous understanding of
second-order logic in the sense that given an infinite set S, the
notion of *every possible* subset of S has an absolute reference (and
not just a reference relative to a particular model of first-order
ZFC). For those of you who (like me or Zermelo: you're in good
company!) think that:
Are there any considerations that rule out the following
(undesirable!) possibility?
Perhaps first-order ZFC is consistent, but second-order ZF is
unsatisfiable. In other words, first-order ZFC only manages to have
models through having 'thin' models, where lots of the sets which
ought to be there aren't. If they were all there, replacement would
just keep on driving up the height of the universe so that one
couldn't ever get a model. Or if you like: it's a truth of
second-order logic that there's no genuinely inaccessible cardinal.
I would like to know why this can't be (or, less ambitiously and
more plausibly, why a modification of our reasons for thinking
first-order ZFC consistent - those that go beyond the rather weak
reason that we haven't found a contradiction yet - also lead to the
conclusion that second-order ZF ought to be satisfiable).
Robert
--
PS I am at present in Berlin, which is why this message is coming
from a gmx address. But you can reply to my usual
<Robert.Black at nottingham.ac.uk>.
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