The question of which set theories satisfy the existence property turned out to be particularly difficult to solve. (Friedman and Scedrov 1985) used Kripke models to show that IZF (that is, the system with collection) does not have the EP, while as mentioned above, the system IZF\(_{Rep}\) (which has replacement in place of collection) does have the EP. This prompted Beeson to pose the question [Beeson 1985, IX]: