Speaking of Category Theory: what can be more concrete than points and arrows between them. (That's the thing about "abstract" math - it is in fact no more abst

Speaking of Category Theory: what can be more concrete than points and arrows between them. (That's the thing about "abstract" math - it is in fact no more abstract, and is often simpler, than the "concrete" math, especially applied math. That's why someone [don't remember who] said that less talented people should stick to working in pure mathematics.)