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The Yoneda lemma is "arguably the most important result of category theory" (source).
Informally, [the Yoneda Lemma] implies that you can gain information about an object by viewing it from *all* possible angles. That kinda makes sense, right? To put it another way: a mathematical object is totally determined by its relationships to other objects. It's sort of like the old saying, “Tell me who your friends are, and I’ll tell you who *you* are.”Math3ma
[T]he collection of all ways to relate an object to other objects is isomorphic to the object itself.https://news.ycombinator.com/item?id=7715277