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Inspired by the answer to this question, is there a model of $\mathsf{ZFC{-}FA}{+}\exists\{x:x\in x\}$ such that $\{x:x\in x\}$ is not empty and does not belong to itself?

Some context

There is a minor debate in philosophy about the status of so-called hypodoxes, which are usually only defined by instances. Two major instances are: co-Russell's set $\exists\{x:x\in x\}$ and Truth-teller sentence "This sentence is true.". The common theme of these instances is that there is some sort of naive under-determinancy around assertions about them (as opposite to paradoxes, which are over-determined in a sense): does co-Russell's set belong to itself? is Truth-teller sentence true?

If you find the concept of a hypodox confusing and unclear, you are not alone. I am now collecting info on the mathematical determinants of the co-Russell's set belonging (or not belonging) to itself. That is also why I ask this question.

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    Consider a model of ZFC without Foundation but with two Quine atoms... – Hanul Jeon Oct 17 '24 at 14:47
  • @HanulJeon how do I know that the presence of two Quine atom's does not force the self-membership of co-Russell? – Timotej Šujan Oct 17 '24 at 15:58
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    @TimotejŠujan If the model is just the cumulative hierarchy generated by the empty set and two distinct Quine atoms, the co-Russell set contains exactly the two Quine atoms, and being that it has two elements, not one, it is not one of the Quine atoms. – spaceisdarkgreen Oct 17 '24 at 17:39

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