I remember at a certain point early in my mathematical studies learning that the Axiom of Choice is equivalent to the following statement on Cartesian products:



> 
> If $\{ X\_i \}\_{i \in I}$ is any collection of nonempty sets indexed by an index set $I$, then $\prod\_{i \in I} X\_i$ is nonempty.
> 
> 
> 


To me, this settled the question of whether to use Axiom of Choice in practical contexts (although it's still interesting to consider systems of math where it doesn't hold, and the interdependence of various other theorems/results/lemmas/axioms on $AC$).


My first question is:


**Question 1--Is there any similarly fundamental lemma or theorem which depends on the continuum hypothesis or its negation? That is, are there any basic facts in set theory, topology, measure theory, etc. which are (a) "self-evident" and (b) equivalent to $CH$ or $\lnot CH$?**


I would also be interested in hearing if such a statement existed for $GCH$ or its negation $\lnot GCH$, although to me $GCH$ seems "less likely" to be true than $CH$ just because it makes a much broader statement over the class of *all* cardinals, whereas $CH$ is a relatively narrow statement about the relationship of two cardinals $2^{\aleph\_0}$ and $\aleph\_1$.


Currently, the two "simplest" results (that I know of) in this vein that would directly depend on $CH$ or $\lnot CH$ are:


1. [Wetzel's problem](https://en.wikipedia.org/wiki/Wetzel%27s_problem)
2. [Whether or not $\Bbb{R}^\omega$ is normal in the box topology](https://mathoverflow.net/questions/314887/is-it-still-an-open-problem-whether-mathbbr-omega-is-normal-in-the-box-top)


But neither of these seem intuitively true or false, much less so essential that we had better accept them one way or another if we want to get any serious math done in the related field.


I'm aware that attempts have been made to resolve $CH$ one way or another (e.g. [Freiling's axiom of symmetry](https://en.wikipedia.org/wiki/Freiling%27s_axiom_of_symmetry)) that are basically trying to reduce $CH$ to such an obviously true/false statement of general set theory/topology/measure theory. So I have a follow-up:


**Question 2--What seem to be the obstacles to finding such a resolution of $CH$ or $\lnot CH$? That is, why is it so difficult to make concrete and testable statements (i.e. not trivial things like "There exists an element of $2^{2^{\aleph\_0}}$ which is neither countable nor of size $\mathfrak{c}$") dependent on $CH$'s truth or falsity? And, should this difficulty be taken as evidence one way or the other for $CH$? Slash, is it actually considered evidence one way or the other for $CH$?**


For instance: every Borel set is either of size $\aleph\_0$ (if countable) or of size $2^{\aleph\_0}$ (if uncountable). Is our difficulty in constructing a set of intermediate cardinality (as opposed to the ease with which we can construct [a non-measurable set](https://en.wikipedia.org/wiki/Vitali_set)) evidence that no such intermediate-cardinality set exists?


I'll also mention that I take the "Platonic view" of $CH$. That is, I believe that despite the existence of models of set theory where either $CH$ or $\lnot CH$ holds, the statement



> 
> "If $S = 2^\Bbb{N}$ is the set of all subsets of $\Bbb{N}$, then for $A \subset S$ any subset of $S$, either $A$ is countable, or there exists a 1-1 correspondence between $A$ and $S$"
> 
> 
> 


has a canonical and demonstrable true/false answer.



Foundational results dependent on/equivalent to the continuum hypothesis or its negation?
Concerning your first question, there is a simple, if not "self-evident", order-theoretic statement equivalent to $CH$ admitting a generalization equivalent to $GCH$:


* If $L$ is a linear ordering of size $2^{\omega}$, then $L$ embeds every cardinal less than $2^{\omega}$ or $L^\*$ ($L$ reversed) embeds every cardinal less than $2^{\omega}$.


This statement can be read in general terms as follows: In order to arrange $2^{\omega}$ points in a line one cannot bypass a smaller cardinal (in the sense that it will appear directly or reversed).


The following generalization is equivalent to $GCH$:


* For every cardinal $\lambda$, if $L$ is a linear ordering of size $\lambda$, then $L$ embeds every cardinal less than $\lambda$ or $L^\*$ embeds every cardinal less than $\lambda$.


It says, in general terms, that for every cardinal $\lambda$, in order to arrange $\lambda$ points in a line one cannot bypass a smaller cardinal.


It is important to remark that the regularity thus stated is trivially true in the finite realm (for finite $\lambda$), contrary to $GCH$ (there are, in general, many numbers between $n$ and $2^n$). Therefore, it is, at least, a more uniform statement generalizing a fact of the finite realm for which, presumably, we have a more reliable "intuition". This feature is present in the usual axioms of set theory.


**Addendum:**


I have just recalled the paper below which seems to be relevant to your question. It connects $CH$ with a more or less concrete problem of machine learning:


* Shai Ben-David, Pavel Hrubeš, Shay Moran, Amir Shpilka and Amir Yehudayoff, *Learnability can be undecidable*, Nat. Mach. Intell. **1** (2019) 44–48, doi:[10.1038/s42256-018-0002-3](https://doi.org/10.1038/s42256-018-0002-3).


($CH$ is equivalent to a version of learnability)


