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