The Yoneda Lemma (2017)

The thing that fascinates me about the Yoneda lemma is that depending on the concrete category you are working with, it provides a theorem which is mostly nontrivial. Depending on the concrete category it provides the following theorems:

First, we are going to generalize elements in sets. The question the Yoneda lemma answers is how the elements behave in this new category.

Source: hkopp.github.io