 Both Mostowski and Lindström had one additional condition in their definitions of generalized quantifiers: they should not distinguish isomorphic models. Informally, they are “topic-neutral”: the truth of a statement of the form \(\f = Qx,yz(A(x),R(y,z))\), say, in a model \(\M\) doesn’t depend on the particular individuals M consists of. If the individuals of M are mapped in a one-one fashion onto the individuals of another universe \(M'\), and if A and R are mapped accordingly, one obtains an isomorphic model \(\M'\). Isomorphism Closure then says that \(\M\models\f\) iff \(\M'\models\f\).