 In general, Existential Generalization, for example in the form of Łukasiewicz’s rule that if \(\mvdash A\rightarrow B\) then \(\mvdash A\rightarrow(\exists x)B\), does not preserve validity if we allow that \(Fx\) may be made true at a world by an individual that does not exist there. When discussing the Converse of the Barcan Formula, Kripke blamed the Necessitation of open formulas, though suggesting that the problem lies not so much in the rule of Necessitation but rather in the alleged theoremhood of the open formulas to which it is applied, and by extension in the validity of the rules of existential generalization and universal instantiation. Kripke’s discussion of this question is limited, also because his modal systems do not employ individual constants, which would reinstate the possibility of proving both the Barcan Formula and its Converse without employing theorems with free variables. 