Let be the Language of set theory. We define the following The smallest class of formulae which contains all atomic formulae Closed Under Bounded Quantification Closed under , , , A formula is if it is of the form for A formula is if it is of the form for A formula is if it is both and . Definition If is any Theory, and a class of formulae, define to be the class of formulas that are equivalent in to a formula in . Theorem formulas are Absolute for Transitive Models of . Proof By induction on the formula complexity in we prove the statement for . Note that the Transitiveness comes into play when dealing with bounded quantifiers. It is then straightforward to extend the statement for . Corollary formulas are Upwards Absolute for Transitive Models of , while are Downwards Absolute. Corollary formulas are Absolute for Transitive Models of .