| This is probably the most difficult part of this presentation... Suppose we have two functors, F,G: X → Y. A natural transformation φ: F → G is defined when for each object x ∈ X there is an arrow φ(x): F(x) → G(x) in Y, and we have the following property:
That's why it is called 'natural' - it acts consistently with the functors actions on arrows. |