 This sentiment also lies behind Prior’s restricted version \(\textbf{MP}_{Q}\) of MP. As noted above, \(\textbf{MP}_{Q}\) prevents the derivation of \(\exists \sfy\,\sfy=\sfy\) from the likes of \(\exists \sfy\,\sfy=\sfa\). But if \(\exists \sfy\,\sfy=\sfy\) were provable, then, since it is wholly general, it would follow by \(\textbf{Nec}^{**}_{Q}\)that it is strongly necessary, \(\Box\exists \sfy\,\sfy=\sfy,\) that is, informally put, it would be a theorem of Q that, in every possible world, there is at least one thing. Again, though, that there couldn’t be an empty world, that it couldn’t have been the case that nothing exists, is a substantive metaphysical thesis that logic is better off leaving undecided. Prior’s restriction in \(\textbf{MP}_{Q}\) ensures that this is in fact the case in Q.