In a prior post I raised the question of how can there be UNCOUNTABLY
many reals each two of which are ONLY discriminated from each other on
Finite basis, and yet we have only COUNTABLY many distinct Finite
initial segments of reals?
What is meant by discrimination on finite basis is the following:
For all reals r1,r2 ( r1 is discriminated from r2 on finite basis <->
Exist n. n is a natural number & Exist d_n, k_n: d_n is the n_th digit
of r1 & k_n is the n_th digit of r2 & d_n =/= k_n)
In other words we know that the capacity for finite discrimination is
Countable, so how come we have uncountably many reals that are
finitely discriminated? It looks as if we are having more reals than
what we could have?
The argument that since we can only distinguish Countably many finite
initial segments of reals then we must have countably many reals so
distinguished is what I call as the 'distinguishability argument'. It
is an argument of Intuition so far as no formalization of it has been
put so far.
The intuitive background of this is found at the head post of that
posting, see below link:
https://groups.google.com/group/sci.logic/browse_thread/thread/9f925ab94b369fad?hl=en
I raised four possible responses to this:
(1) To say that the formal proof of Cantor is clear and exact in
formal terms, but the distinguishability argument is clear on
intuitive level but has not been verified in formal terms, so
accordingly we have the option of saying that Infinity do not copy
intuitions derived from the finite world, and deem the result as just
counter-intuitive but not paradoxical. I think this is the standard
approach.
(2) To say that the distinguishability argument is so clear and to
accept it as a proved result despite the possibility of verifying it
at formal level or not, and also maintaining that Cantor's proof is
very clear and valid, and so we deduce that we have a genuine paradox
(antinomy) that resulted from assuming having completed infinity, and
thus we must reject having completed infinity. That's what WM is
saying
(3) To consider uncountability of the reals to be FALSE by
interpreting the universal quantifier in Cantor's argument to range
over *elements* of the universe of discourse, and thus it doesn't
cover all available functions (which are subsets of the universe of
discourse), so there is a bijection between the reals and the naturals
that is simply missing from being among the *elements* of the universe
of discourse. So the reality of that matter is that the set of all
reals is countable but discourse misses the necessary bijection. This
is the interpretation of having a countable model of a theory that
proves the existence of uncountably many objects. It is a consequence
of Skolem's arguments. The justification for that is that if consider
the universal quantifier in Cantor's argument to range over subsets of
the universe of discourse, then we are at second order logic which is
not known to support a proof system and so it can barely be seen as a
kind of logic or something that we prove things after. So we are
better with at first order. If we use Henkin semantics to explain the
second order quantifier then we'll end up by the same argument of
Skolem. So Henkin semantics for second order or first order semantics
both ensure having a countable model of any theory, and since proof
theory by ordinal analysis depends on constructiveness within
countable limits, then we only need to stipulate the existence of
countable models since it is those the ones that are both provable and
economic in the sense of reductionist approach reminiscent of Ockam's
razor, that is if we can do the job with less so why demand more. And
since there is no logical argument to force us to adhere to
uncountability without assuming it first, so why adhere to?
(4) To adopt a very radical perspective about sets, a perspective that
only admits sets that are definable in a parameter free manner to
exist, and accordingly to consider countability of all finite initial
segments of reals to be FALSE, i.e. to say that we have uncountably
many finite initial segments of reals and as well we have uncountably
many reals. This clearly preserves congruity of the argument, but it
requires justification. The idea is that the alleged bijection between
the finite initial segments of reals and the set N of all naturals is
NOT parameter free definable and so this bijection does not exist, and
it is false to say that it is. This claim only accepts infinite sets
to exist if there is a parameter free formula after which membership
of those sets is determined, so if there is non then it doesn't accept
the existence sets that are not parameter free definable.
MY PERSONAL RESPONSE:
I personally favor the standard response which is the first response.
However I think that further insights to address the intuitive
challenge presented by the distinguishability argument is warranted.
I'm not sure if the fourth response is possible, any proof of the
alleged bijection (between the naturals and the set of all initial
segments of parameter free definable reals) being parameter free
definable would abolish that response. However even if a proof is
presented of the alleged bijection being not parameter free definable,
still there is a strong sense to compel us to agree to the set of all
finite initial segments of reals being countable, and to get rid of
that just for the sake of parameter free definability doesn't seem to
be plausible at all. So all in all the fourth response is not
plausible that if not impossible.
The second response is too radical and it upgrades 'counter-
intuitiveness' to the level of antinomy without putting forwards any
formal proof, and thus it is just an assertion that have some
intuitive appeal but it is unbaked so far by any rigorous formal
proof.
The third response though rises within many contexts of formal
theories in first order languages, however it seems to be an
artificial midway fix between intuition and formality.
Zuhair