 Isom, i.e., topic neutrality, is standardly seen as at least a necessary condition for being a logical  constant.[30]  It is possible to distinguish logicality from constancy in the earlier mentioned sense of meaning the same over different universes. For one thing, logicality is a property that ought to be closed under definability, whereas it is not at all clear that constancy should be similarly closed. Note, for example, that the class of Ext quantifiers is not closed under first-order definability. More precisely, it is closed under the usual Boolean operations, but not under inner negation and hence not under taking duals, where the inner negation of a type \({\langle}1{\rangle}\) quantifier Q is defined by \((Q\neg)_M(A) \Leftrightarrow Q_M(M\!-\!A)\), and the dual by \(Q^d = \neg(Q\neg)\). For example, \(\exists^d = \forall\).