When does every definable nonempty set have a definable element?
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The assertion that every definable set has a definable element is equivalent over ZF to the principle $V=\text{HOD}$, and indeed, we prove, so is the assertion merely that every $\Pi_2$-definable set has an ordinal-definable element. Meanwhile, every model of ZFC has a forcing extension satisfying $V\neq\text{HOD}$ in which every $\Sigma_2$-definable set has an ordinal-definable element. Similar results hold for $\text{HOD}(\mathbb{R})$ and $\text{HOD}(\text{Ord}^\omega)$ and other natural instances of $\text{HOD}(X)$.
[1] Joel David Hamkins,et al. Pointwise definable models of set theory , 2011, The Journal of Symbolic Logic.
[2] Joel David Hamkins. A simple maximality principle , 2003, J. Symb. Log..