From: Bruno Marchal <[EMAIL PROTECTED]>
Subject: Re: Why is there something rather than nothing?
Date: Thu, 20 Nov 2003 12:57:55 +0100

At 18:30 19/11/03 -0500, Jesse Mazer wrote:
Does anyone know, are there versions of philosophy-of-mathematics that would allow no distinctions in infinities beyond countable and uncountable? I know intuitionism is more restrictive about infinities than traditional mathematics, but it's way *too* restrictive for my tastes, I wouldn't want to throw out the law of the excluded middle.

I don't know. In general people who accept
the uncountable accept most big cardinals.
This follows from the fact that in most set
theories you can prove Cantor theorem
which say that card(power of A) is always
strictly bigger than card(A).

But as I said earlier, the power set of a countable set would include plenty of sets that could not be defined using any sort of finite description, and I'm skeptical about the "existence" of such objects in a Platonic sense. Perhaps what I'm talking about would be a form of "mathematical constructivism", although I don't know how "constructible" is usually defined, and whether it would allow for noncomputable numbers that can still be defined in terms of some finite program of an oracle-machine whose "level" can itself be finitely defined (see my post at[EMAIL PROTECTED]/msg04809.html for what I mean by different 'levels' of oracle-machines, I'm not sure if my terminology is correct here although I've seen similar ideas expressed before).

On second thought though, if I am only willing to admit mathematical objects with finite descriptions, then I should probably not even distinguish between countable and uncountable sets, since the collection of all possible finite descriptions must be countable. But there is another sense in which it makes sense to distinguish two different types of infinity here. To prove that a set is countable, one must find a function mapping the positive integers to the members of the set, with every member corresponding to one integer. But if only mathematical objects with finite descriptions are permitted, this should go for functions mapping integers to mathematical objects as well; and it's relatively easy to see that no finitely-describable function would include *every* object with a finite description (but no meaningless descriptions which don't pick out mathematical objects at all), using a diagonal argument. Suppose we just want to come up with an exhaustive list of all the finitely-describable reals, and we have some candidate function which maps every positive integer to a real with a well-defined finite description ('well-defined' meaning that there is no ambiguity about what number the description picks out--'a number bigger than 4' would not be a well-defined description, for example, since it doesn't pick out a unique real). If the function F is itself clearly defined, so there is no ambiguity about what real a given integer is mapped to, then a description like "the real number formed by changing the nth digit of the binary expansion of the real number that maps to integer n under function F" should itself be a well-defined finite description which picks out a real number that differs from every number on the list by at least one digit.

So, although the set of all well-defined finite descriptions must clearly be "countable" in the traditional sense where arbitrary mappings are allowed, it is not countable if only finite-describable mappings are allowed, although it can easily be shown to be smaller than another countable set, namely the set of all finite descriptions without regard for whether they are "well-defined" or not (just list all strings of english symbols in lexical order, although most of these will be completely meaningless like "xhh{we2lkjk4j5j", and will thus not pick out any particular real number). The basic problem here is that the notion of what it means for a description to be "well-defined" can never be stated in terms of a formal rule--this is reminiscent of the fact that no formal rules can encompass our understanding of arithmetic, that no matter what axioms we use there will always be theorems that we can prove to be true in ordinary-language proofs which cannot be proved true or false by the axiomatic system. These ordinary-language proofs will nevertheless be seen as quite rigorous by any mathematicians who are Platonists about arithmetic--I believe Godel used such a proof to show that the Godel statement G for the axioms of Peano arithmetic (or any other axiomatization of arithmetic) must in fact be a true theorem even though it's not provable using the axioms, as long as we interpret the symbols to refer to our mental model of arithmetic (see the discussion about Platonic 'models' in math at[EMAIL PROTECTED]/thrd2.html#04463 for more on this).

Jesse Mazer

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