So for any two categories $ \mathscr{A} $ and $ \mathscr{B} $ , there is a category whose objects are the functors from $ \mathscr{A} $ to $ \mathscr{B} $ and whose maps are the natural transformations between them. This is called the functor category from $ \mathscr{A} $ to $ \mathscr{B} $ , and written as $ \ftrcat{\mathscr{A}}{\mathscr{B}} $ or $ \mathscr{B}^\mathscr{A} $ . Example: Let $ 2 $ be the discrete category with two objects.