(Chapter Two is left as an exercise for the student?)
The comment “What an idiot I was,” I thought. “That was just an axiom, it is called commutativity. One doesn’t prove axioms.” is interesting. What's chosen as an axiom, and why, is an advanced question. Unless you get into foundations of mathematics, that question is seldom addressed. It's way beyond most pre-college math teachers. It's the sort of question that occurs to smart kids, but there's no easy answer you can give them. The usual answers are theological, and boil down to "shut up, kid". Here's a discussion on Stack Exchange of that subject: http://math.stackexchange.com/questions/127158/in-what-sense...
(If you get into automatic theorem proving, you have to address such issues head-on. Adding an inconsistent axiom can create a contradiction and break the system. This leads to constructive mathematics, Russell and Whitehead, Boyer and Moore, and an incredible amount of grinding just to get the basics of arithmetic and number theory locked down solid. In constructive mathematics, commutativity of integer addition is a provable theorem, not an axiom.
I once spent time developing and machine-proving a constructive theory of arrays, without the "axioms" of set theory. The "axioms" of arrays are in fact provable as theorems using constructive methods. It took a lot of automated case analysis, but I was able to come up with a set of theorems which the Boyer-Moore prover could prove in sequence to get to the usual rules for arrays. Some mathematicians who looked at that result didn't like seeing so much grinding needed to prove things that seemed fundamental. This was in the 1980s; today's mathematicians would not be bothered by a need for mechanized case analysis.)