 Closure: For any formula \(\phi(x)\), there is a set \(C\) such that \(x \in C \equiv [\forall y\forall z(\phi(z) \rightarrow z \in y) \rightarrow x \in y\)]; \(C\) is the intersection of all sets which include all objects which satisfy \(\phi : C\) is called the closure of the class \(\{x \mid \phi(x)\}\).