From "The Yoneda Perspective", a 2017 essay by Tai-Danae Bradley, discussing the Yoneda Lemma of Category Theory:

... mathematical objects are completely determined by their relationships to other objects ...

... and:

... the properties of a mathematical object are more important than its definition ...

... and in the remarks on that essay, a comment by David Roberts:

Q: "What is the Yoneda lemma? And if it's just a lemma then - my gosh - what's the theorem?"A: "All of them." |

*(cf Tai-Danae Bradley's blog and her 2018 notes "What is Applied Category Theory?"...)* - * ^z* - 2018-10-03