Hom Functor - Yoneda's Lemma

Yoneda's Lemma

Referring to the above commutative diagram, one observes that every morphism

h : A′ → A

gives rise to a natural transformation

Hom(h,–) : Hom(A,–) → Hom(A′,–)

and every morphism

f : B → B′

gives rise to a natural transformation

Hom(–,f) : Hom(–,B) → Hom(–,B′)

Yoneda's lemma implies that every natural transformation between Hom functors is of this form. In other words, the Hom functors give rise to a full and faithful embedding of the category C into the functor category SetC (covariant or contravariant depending on which Hom functor is used).