https://www.zhihu.com/question/31823394/answer/53592663?from=timeline
Key Points:
- Category and Functor are above the underlying algebraic structures (Set, Group, Ring, Vector Spaces, etc), study the relations between these structures.
- Early 19 CE mathematicians before “Category Theory” already knew there is 1:1 mapping between the Field Extension and Galois Group.
Treat Structures and Relation between them (Functors) on equal f
