On the Existence and Disjunction Properties in Structural Set Theory

Abstract

We formulate a definition of the existence property that works with "structural" set theories, in the mode of ETCS (the elementary theory of the category of sets). We show that a range of structural set theories, when formulated using constructive logic, satisfy the disjunction, numerical existence, and existence properties; in particular, intuitionist ETCS, formulated with separation and Shulman's replacement of contexts axiom, satisfies these properties. As a consequence of this, we show that, working constructively, replacement of contexts is strictly weaker than collection.