This constraint is in the spirit of those principles of set theory—most notably, reflection principles—which aim to capture the pretheoretic idea that the universe of sets is so rich that it cannot “be described from below”; more precisely, it asserts that any tenable conception of truth must respect the idea that the universe of sets is so rich that truth (or even just Π2-truth) cannot be described in some specifiable fragment. (Notice that by Tarski's theorem on the undefinability of truth, the truth constraint is trivially satisfied by the standard conception of truth in set theory which takes the multiverse to contain a single element, namely, V.) 