"The cornerstone of Category Theory is the Yoneda lemma. It asserts that a category C may be embedded in the category C^\wedge of all contravariant functors from this category to the category Set of sets, the morphisms in Set being the usual maps. This allows us, in some sense, to reduce Category Theory to Set Theory. The Yoneda lemma naturally leads to the notion of representable functor, and in particular to that of adjoint functor."