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The "true" value of $2^{\aleph_0}$ is one of the most fundamental open questions of mathematics and its philosophy. Hundreds of set theoretic results during the last century don't say anything more than "it could be everything under current weak foundation of mathematics". In fact Hilbert's first question is still open and even more open than the first time which he announced it because "How many real numbers do we have "really"?" is a completely different question from "How many real numbers "can" we have?" The set theoretic results tells many things about the last question but a few about the first one which is Hilbert's first question.

A well-known conjecture about the value of $2^{\aleph_0}$ is Continuum Hypothesis ($\text{CH}$) which says $2^{\aleph_0}=\aleph_1$.

Some set theorists (e.g. Goedel) who mainly believe on ontological maximalism, think $\text{CH}$ is false. Some others who believe in smaller universe of mathematical objects think $\text{CH}$ is true. Recently a third point of view is growing up by some set theorists like Hamkins and Feferman. It says the value of continuum is "indefinite" and the "true value" of $2^{\aleph_0}$ doesn't exist at all.

The following is quoted from Wikipedia.

Solomon Feferman (2011) has made a complex philosophical argument that $\text{CH}$ is not a definite mathematical problem. He proposes a theory of "definiteness" using a semi-intuitionistic subsystem of $\text{ZF}$ that accepts classical logic for bounded quantifiers but uses intuitionistic logic for unbounded ones, and suggests that a proposition $\phi$ is mathematically "definite" if the semi-intuitionistic theory can prove $(\phi \vee \neg\phi)$. He conjectures that $\text{CH}$ is not definite according to this notion, and proposes that $\text{CH}$ should therefore be considered not to have a truth value.

Question: What is known about Feferman's conjecture on indefinite value of continuum? References on the philosophical arguments about this conjecture are also welcome. (The only paper which I am aware of is Koellner's article.)

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  • $\begingroup$ For further arguments on the true value of $2^{\aleph_0}$ (which is not what I am searching for here) see this related MO post: Solutions to the Continuum Hypothesis $\endgroup$
    – user45939
    Commented May 10, 2014 at 1:32
  • $\begingroup$ Is there an intuitionistic model coming from topos theory in which $\text{CH}\vee\neg\text{CH}$ is not true? If so, would this establish Feferman's conjecture? $\endgroup$
    – Joel David Hamkins
    Commented May 10, 2014 at 1:42
  • $\begingroup$ @JoelDavidHamkins I don't know. I am new in this point of view. Personally I strongly believe in ontological maximalism also I like plural Platonism because it is a fundamentally new approach. As a consequence I am searching for a consistency between ontological maximalism and plural Platonism. $\endgroup$
    – user45939
    Commented May 10, 2014 at 1:53
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    $\begingroup$ I think your question is a good one, but I disagree with your introduction. With the caveat that I'm not a set theorist, it has always seemed to me that $2^{\aleph_0}$ is a perfectly definite number — the question that ZF doesn't answer is rather "how big is $\aleph_1$?". $\endgroup$
    – Theo Johnson-Freyd
    Commented May 10, 2014 at 5:14
  • $\begingroup$ @JoelDavidHamkins I don't know this for CH, at least for the AC, there must be one, since AC+extensionality+replacement imply TND. So I wouldn't be surprised if something similar holds for the AC. Anyway, I think the Question how many reals "really" exist is strange. I don't know much about philosophy, but in standard classical mathematics, reals and sets are basically defined by the axioms. And the question boils down to whether there exists a subset of the reals which has a surjection into N but no bijection into R and N. And this is just a question of definition. $\endgroup$
    – Christoph-Simon Senjak
    Commented May 10, 2014 at 14:35

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Feferman has posted an updated version of his paper:

http://math.stanford.edu/~feferman/papers/CH_is_Indefinite.pdf

Towards the end, he mentions that Michael Rathjen has proved Feferman's conjecture, that at least in Feferman's proposed formalization, CH is definitely not definite. The formalization itself is also elaborated a bit more, and he refers to another 2014 publication of his.

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Recently M. Rathjen have proved, that CH (Continuum Hypothesis) is a indefinite problem in system SCS. In my last work I prove the same for two-sorted intuitionistic Zermelo-Fraenkel set theory ZFI2C with the Collection. This theory is much stronger than SCS. In fact, the classical theory ZF2 = ZFI2C+LEM by some variant of Friedmans version of Goedels negative translation. Of course, the same holds for one-sorted theory ZFIC with the Collection schema. So, CH is an indefinite problem in a rather strong sense.

Sincerely yours, Alexey G. Vladimirov

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  • $\begingroup$ Is your last work available online? $\endgroup$
    – Pedro Sánchez Terraf
    Commented Aug 15, 2018 at 18:09

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