Fibrations of predicates and bicategories of relations

We reconcile the two different category-theoretic semantics of regular theories in predicate logic. A 2-category of regular fibrations' is constructed, as well as a 2-category of regular proarrow equipments’, and it is shown that the two are equivalent. A regular equipment is a cartesian equipment' satisfying certain axioms, and a cartesian equipment is a slight generalization of a cartesian bicategory. This is done by defining a tricategory Biprof whose objects are bicategories and whose morphisms are category-valued profunctors, and then defining an equipment to be a pseudo-monad in this tricategory. The resulting notion of equipment is compared to several existing ones. Most importantly, this involves showing that every pseudo-monad in Biprof has a Kleisli object. A strict 2-category of equipments, over locally discrete base bicategories, is identified, and cartesian equipments are defined to be the cartesian objects in this 2-category. Thus cartesian equipments themselves form a 2-category, and this is shown to admit a 2-fully-faithful functor from the 2-category of regular fibrations. The cartesian equipments in the image of this functor are characterized as those satisfying certain axioms, and hence a 2-category of regular equipments’ is identified that is equivalent to that of regular fibrations. It is then shown that a regular fibration admits comprehension for predicates if and only if its corresponding regular equipment admits tabulation for morphisms, and further that the presence of tabulations for morphisms is equivalent to the existence of Eilenberg–Moore objects for co-monads. We conclude with a brief examination of the two different constructions of the effective topos, via triposes and via assemblies, in the light of the foregoing.