[FOM] axioms for potential infinity and for actual infinity?

Maarten McKubre-Jordens maarten.jordens at canterbury.ac.nz
Sun Mar 8 16:12:04 EDT 2015

Joao Marcos wrote:

> Hummm... what if one replaced ZF's *Axiom of Infinity*, that
> postulates the existence of inductive sets, by an axiom that
> postulated the existence of
> sets equipped with endofunctions that are injective but not
> surjective?  It is well-known that the latter sets (known as
> *Dedekind-infinite*) are infinite in the usual sense, while proving
> the converse demands some extra technology, such as AC? (countable
> choice).
> This is all standard, but I wonder if anyone has tried to use it as a
> way of rescuing the age-old philosophical distinction between actual
> and potential infinity.  No doubt, if Dedekind-infinite is to mean
> "actual", to give the traditional Axiom of Infinity a distinct
> "potential" flavor it might make sense to move first to a *predicative
> version of ZF* (which will often weaken also the powerset axiom to
> some sort of "axiom of exponentiation"), so that the "existence"
> postulated by the axiom is given a constructive reading.  Does anyone
> know if *infinite* implies *Dedekind-infinite* in Constructive ZF with
> Dependent Choice?

I don't know if this is a help, but there's a paper from Lubarsky and Rathjen where they construct a Kripke model that shows that, without Dependent Choice, the Dedekind reals form a proper class whereas the Cauchy reals form a set. The equivalence of the latter two in Bishop-style constructive mathematics can be explained by the acceptance of Dependent Choice in that theory. The reference is:

Robert S. Lubarsky & Michael Rathjen, 'On the constructive Dedekind reals', Logic and Analysis 1 (2):131-152 (2008).

Based on that, my guess would be that infinite does imply Dedekind-infinite provided one has Dependent Choice.


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