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There is a natural bijection between functors and morphisms in , namely the map that sends a functor to the image in of the morphism in .
is not itself a map from to ; it is a category (the arrow category itself) whose objects are maps (functors) from to . So an object of is a functor from to .
And what does this functor consist of? has two objects, and ; so we need two objects of , and . Similarly, has three morphisms, , , and ; so we need three morphisms of , , , and . Since the functor must preserve identities, we need and , so the data remaining is precisely that in the morphism . It now remains to check that the functor preserves composition, but it does.
So a functor from to consists of precisely the same information as a morphism in .
We still have to check that the morphisms in (which are natural transformations) correspond in this way to commutative squares in , as claimed; I leave that to you.
One problem with this article is that it still shows our early tendency to give everything the slickest definition possible. I've rewritten it in a way that I think is more comprehensible.
yes, I see. While the previous definition is good for the brain if one has faith enough to pursue it, it does leave one with the feeling that in CT the easy is explained by the more complex. With your new definition the intuitive is explained first, then one gets an identity statement with a purer, though more complex way of stating things.
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