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$Define: X \text { is cardinal definable} \iff \\\exists \text { cardinal } \kappa \, \exists \text { cardinals } \lambda_1,.., \lambda_n <^\rho \kappa \ \exists \phi : \\ X=\{ y \in V_{\rho(\kappa)} \mid \phi^{V_{\rho (\kappa)}} (y,\lambda_1,..,\lambda_n)\}$

Where: $\lambda_i <^\rho \kappa \iff \rho(\lambda_i) < \rho(\kappa)$, and $\rho$ is the rank function; and "cardinal" is defined after Scott's.

Now, is the principle that every set is cardinal definable consistent with ZF?

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Yes, assuming ZF is consistent it is, because it follows from ZFC + V=HOD. This is because the ordertype of the class of (ordinal-)cardinals is just that of the ordinals. That is, if $X$ is definable over $V_\alpha$ from ordinal parameter $\beta<\alpha$, then $X$ is definable over $V_{\aleph_\alpha}$ from ordinal parameter $\aleph_\beta$, since $\xi\mapsto\aleph_\xi$ restricted to $\alpha$ is also definable over $V_{\aleph_\alpha}$.

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  • $\begingroup$ But what about the following variant (or was this maybe what you were really thinking about?): Is ZF + $\neg$AC + "every set is cardinal-definable" consistent, where of course now the cardinals need not be wellorderable? $\endgroup$
    – Farmer S
    Commented Dec 26, 2021 at 10:17
  • $\begingroup$ Yes, that was in my mind as a second result, I wanted to know the proof of the more general result, then this other question would follow. Thanks $\endgroup$ Commented Dec 26, 2021 at 12:26

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