Boolean category (Rev #7) in nLab

Contents

Definition

A Boolean category is a coherent category (such as a topos or pretopos) in which every subobject has a complement, i.e., for any monomorphism A↪XA\hookrightarrow X there is a monomorphism B↪XB\hookrightarrow X such that A∩BA\cap B is initial and A∪B=XA\cup B = X. Therefore, the subobject lattice Sub(X)Sub(X) of any object XX is a Boolean algebra.

Properties

Any Boolean category is, in particular, a Heyting category and therefore supports a full first-order internal logic. However, unlike that of an arbitrary Heyting category, the internal logic of a Boolean category satisfies the principle of excluded middle; it is first-order classical logic.

Related entries

• pretopos
• Heyting category
• De Morgan category?
• positive category
• Boolean hyperdoctrine