On completeness and cocompleteness in and around small categories Original Research Article

The simple connection of completeness and cocompleteness of lattices grows in categories into the Adjoint Functor Theorem. The connection of completeness and cocompleteness of Boolean algebras — even simpler — is similarly related to Paréʹs Theorem for toposes. We explain these relations, and then study the fibrational versions of both these theorems — for small complete categories. They can be interpreted as definability results in logic with proofs-as-constructions, and transferred to type theory.