FOM: the Urbana meeting
Martin Davis
martin at eipye.com
Tue Jun 20 16:35:22 EDT 2000
I was on the Program Committee for the Urbana ASL meeting, and the
committee was enthusiastic about the proposed panel on the need for "new
axioms". Some time ago in a telephone conversation, Harvey told me that I
am an "extreme Platonist". Being a great fan of Harvey's work on the
necessary use of large cardinals, I took his comment quite seriously and
began to wonder. Is that really me?
In this posting, I will comment on Harvey's presentation in Urbana and on
what my own views really are. In his talk, Harvey said that he was bringing
some very bad news, but also some good news. The bad news is that set
theory in particular and foundational studies in general are on hard times
and that unchecked, things would only get worse. The good news is that
Harvey's new Boolean Relation Theory will save the day: Because it is a
single appealing theme, whose necessary methods range from what
mathematicians are used to, all the way through Mahlo cardinals, and
eventually all the way up the large cardinal hierarchy, mathematicians will
be led to accept these methods because they will see that they are needed
to solve problems that interest them. He also implied that the traditional
set theory community is on the wrong track. I certainly applaud Harvey's
program, but I assume that since Harvey is devoting himself to this
program, he believes that the results he gets are TRUE. What I don't
understand is what he'll tell mathematicians who want to know why they
should believe this.
The question of whether people could hope to use rational methods to
investigate completed infinitudes is an old one, and I know only a little
of the history. Gauss famously insisted that infinity in mathematics is
always just a manner of speaking, referring to a limiting process. Much
earlier Leibniz had embraced completed infinities while insisting there
could be no infinite cardinal numbers (because of the "paradox" of a whole
having the same cardinality as one of its parts). Cantor boldly ventured
forth and developed a coherent mathematics of the infinite. Now, I have no
belief that there are a priori methods by which we can guarantee the
validity of this undertaking. Goedel spoke of an "organ" by which we intuit
abstract concepts; I haven't the least faith in the existence of such an
organ. What I believe to be true (and it is hard for me to see how it can
be doubted) is that our reasoning faculty that presumably evolved to help
our hunter/gatherer ancestors with mundane tasks is effective over a far
greater range than this purpose would suggest - after all this is what
makes possible such things as science, mathematics, and fiction. I think
the question of to what extent this faculty can be effective in exploring
the infinite is an empirical question that can only be decided by trying
and analyzing the results. Do we obtain a coherent picture? Or does it all
dissolve in vagueness and contradictions? (I think this view is close to
Maddy's "naturalism".)
From this point of view, the work of set-theorists has been crucial in
suggesting that it is the former that is the case. The use of PD in
providing an elegant theory of the projective hierarchy and the discovery
that PD is implied by large cardinal axioms encourages the view that one is
dealing with a situation where there is an objective fact-of-the-matter
with respect to the propositions being studied. The more recent work
showing that consistency strength alone of certain of these axioms suffices
to determine the truth values of sentences of given complexity, further
enhances this perception. I am at a loss to understand why Harvey thinks
that this work and his are at cross-purposes; it is clear to me that each
needs the other: Harvey to show concretely that the higher infinities have
specific interesting consequences way down, the set-theorists to map out
the infinite terrain and provide a convincing case for a coherent robust
state of affairs.
Sol Feferman is a brilliant learned expositor, his original article for the
MONTHLY on the need for new axioms is very beautifully written, and his
talk in Urbana maintained the same high standard. Sol maintains that CH is
inherently vague and for that reason it is pointless to expect that the
question will ever be resolved. This conclusion is not surprising since he
finds that he does not believe that the concept of the continuum (or
equivalently, the power set of omega) is well-defined. Since CH is about
the cardinality of this very set, naturally, for someone with Sol's
beliefs, CH can have no determinate truth value. Sol's belief about the
continuum has been held by such great mathematicians as Brouwer and Weyl,
and so he is certainly in excellent company. However what bothers me is how
his conclusion will be received by readers of his MONTHLY article with
little training in foundations. In my experience, such mathematicians
presume that the Goedel-Cohen independence results have settled the matter
about CH, imagining that it is quite like the situation with the parallel
postulate, and there is nothing more to be said. Such folk hearing Sol's
conclusion about CH will likely nod their heads. But typically, they work
with the continuum every day, and by no means are likely to share Sol's
belief that it is a questionable concept, the belief on which his
conclusion is based.
My own presentation in Urbana was part of a panel on the history of logic
in the 20th century. I chose to talk about what I like to call Goedel's
Legacy. Goedel emphasized that the right way to think about the
incompleteness phenomenon for Pi-0-1 sentences, is that such sentences are
seen to be true by proceeding to a higher type, that is, by climbing up a
level in the cumulative hierarchy. From this point of view the explorations
of the higher transfinite serve to provide possible strong axioms for
realizing Goedel's idea. Of course the question remains: are any problems
of genuine mathematical interest likely to be examples of the
incompleteness phenomenon, even such problems of central importance as the
Riemann Hypothesis (as Goedel ventured to suggest). In my talk, I suggested
that on this question, interested people could be divided into three
classes: optimists (people who think that such interesting undecidable
propositions will be found - or even, are already being found), skeptics
(people who think that Gödel incompleteness will not affect propositions of
real interest to mathematicians), and pessimists(thinks that even if there
are such propositions, it will be hopeless to prove them). In replying to a
question from Dana Scott, I admitted that I am an optimist.
Martin
Martin Davis
Visiting Scholar UC Berkeley
Professor Emeritus, NYU
martin at eipye.com
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http://www.eipye.com
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