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Countabilism is, roughly, a family of standpoints inclusive of:

  1. There is one infinite proper set, of size ℵ0, and one infinite proper class, ℵ00. (See about e.g. "pocket-sized" and Vopenkan set theory in the SEP. For those constructivists/intuitionists for whom even ℕ is not a "completed totality," it trivially follows that the powerset of ℕ is not a "completed totality" either, but so see also Storer[10] as well as Barton and Friedman[21] for similar or sometimes even equivalent propositions.)
  2. There is one infinity, as a "completed totality," only, but it is not a set, it is a proper class (or if it is a set, it is a universal one), and this infinity is that of countability already. (Note that the powerset axiom scheme (for infinity) is incompatible with this system.) Suggested by the thematism of Skolem's paradox.

We might parse (1) as weak countabilism, (2) as strong.

At any rate, "everyone knows that a set is infinite if and only if it can be put into a one-to-one correspondence with a proper subset of itself," except nowadays we know that this is not so (actually, this has been known by some for a while longer...), the if-and-only-if parameter is too much, for there is the apparent logical possibility of amorphous sets, which are Dedekind-finite but otherwise infinite. More generally, in set theories open to the possibility of choiceless sets, there are many simple-enough-structure infinite sets that are eventually available that, while not commensurate with the alephs, yet directly (or quantitatively) at least echo the qualitative simplicity of the zeroth aleph. (For more on amorphous and other choiceless sets, see Truss[95] as well as Harrison-Trainor and Kulshreshtha[22].) My question is: if the conception of an infinite set is generically ambivalent in this manner, does countabilism lose some of its apparent plausibility, at least to the extent that strong countabilism is seriously (modally) undermined, while weak countabilism needs to be reconceived as admitting, alongside the zeroth aleph, at least one or so amorphs or other choiceless cardinals, with the proper class of the set world then becoming something like ℵ0A or even 20A (for A (co)amorphousc)?


cLet W be the class of well-ordered sets and -W be the class of sets that are not well-ordered. Then let C, for coamorphous, be the complement in -W of A (for A amorphous).

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    This seems to depend on what "countabilists" think about the axiom of choice. Countable choice is enough to rule out amorphous sets, and it is not enough to infer the law of excluded middle and various pathologies. If countable choice is "apparently plausible" (and if they go for countable infinity, why not?) then maybe their conception of the infinite is not so "generically ambivalent".
    – Conifold
    Commented Mar 18 at 12:22
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    A question and some thoughts. First I am not sure I understand what "generically ambivalent" means in your question? Then, I think countabilism is more of a model-belief than a theory-belief. It suggest what the set-theoretic world is, not what its rules are. One the other hand, the possibility of amorphous set is a theory-oriented result: namely such sets are consistent with ZF. So its not clear how to make the two things work together: countabilism is simply not consistent (as a model) with ZF. Also, any fo theory has non-standard models with which this argument could be repeated.
    – Johan
    Commented Mar 19 at 13:07
  • @Johan there must be a more general definition of infinity than Dedekind-infinite, but then it turns out that this general definition has "simplest" satisfiers in not only countable and amorphous infinity, but other, incommensurate, choiceless "base cases" as well. So it seems as if one can't appeal to the mere simplicity of countable infinity to justify countabilism, since either there are multiple alternatives to that, or there are incommensurate measures of simplicity too, etc. Whether countabilism is more a model or a theory perhaps depends on the countabilist in question... Commented Mar 19 at 17:06
  • Kristian is BACK. We should talk man I have been learning a lot about logic lately as well. You’ve clearly been plowing ahead in your learning odyssey. Commented Mar 22 at 13:50
  • From what I can tell, the ethos behind Countabalism stems largely from the fact that most of "standard" mathematics is concerned with sets of countable cardinality, or sets with Cardinality of the continuum. Unless amorphous sets appear in the construction of "standard mathematics" I dont believe their conceptual possibility is of concern to countabalists, similar to how the conceptual possibility of the Powerset Axiom, isn't posed as an issue. The mission of countability, to simplify the universe of sets seems entirely at odds with the notion of amorphous sets. Commented Mar 23 at 4:56

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A strong countabilist would likely choose to use at most PA or, say, some theory of hereditarily finite sets, so no 'infinite' objects of any kind to worry about

For a classical weak countabilist, possibly at most full second order arithmetic, maybe some 'faux' two-sort first-order subtheory, but one strong enough to prove the nice theorem

For any infinite set X, there exists a function f such that ∀k∀m (k < m -> f(k) < f(m)) and ∀n(n ∈ X <-> ∃m(f(m) = n))

available already at rather weak theories

Finally, a constructive/predicative/etc. weak countabilist may use some adequate type theory, with pertinent restrictions on construction of function types, etc., etc.

So, in any case, they'd be happily living with/in their chosen theories, which disallow the existence of such amorphous/ill-structured/pathological infinite objects by design, and would not care at all that these may exist in some other 'suspicious' theories

For anyone else, the possibility of amorphous infinite sets may well be evidence against the plausibility of Z(F) and related theories

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  • For your last paragraph, see its wikipedia source: Fraenkel constructed a permutation model of Zermelo–Fraenkel with Atoms in which the set of atoms is an amorphous set. After Cohen's initial work on forcing in 1963, proofs of the consistency of amorphous sets with Zermelo–Fraenkel were obtained... Commented Mar 29 at 21:52
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    @DoubleKnot yes, I know, what's your point exactly?
    – ac15
    Commented Mar 29 at 21:56
  • So why you claim "the possibility of amorphous infinite sets may well be evidence against the plausibility of Z(F)"?... Commented Mar 29 at 21:57
  • @DoubleKnot because a theory that produces (possibly) pathological objects may be regarded as not plausible
    – ac15
    Commented Mar 29 at 22:01
  • What if consistency precedes pathology assuming such pathology is well defined?... Commented Mar 29 at 22:07

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