 Recall that a set a is called transitive if every element of an element of a is an element of a. A transitive model is a model \((M,\in)\) such that M is transitive. By the Mostowski Collapsing Lemma (Mostowski 1949) every well-founded model \((M,E)\) of the Axiom of Extensionality is isomorphic to a transitive model. A basic property of transitive models \((M,\in)\) is that if \(a,b\in M\), then \(M\models a\subseteq b\) if and only if \(a\subseteq b\). (If all elements of a that are in M are in b then all elements of a are in b, as by transitivity of M, elements of a are necessarily elements of M.) We could express this by saying that the formula \(x\subseteq y\) is absolute in transitive models. More generally, we say that a formula \(\phi(x_1,\ldots,x_n)\) of the first order language of set theory is absolute relative to ZFC if there is a finite \(T\subseteq \ZFC\) such that for all transitive models \((M,\in)\) of T and all \(a_1,\ldots,a_n\in M\) we have 