I bought Steve Awodey’s book Category Theory (Oxford Logic Guides, Clarendon Press, 2006) when it first came out. Awodey says that his book is aimed, inter alia, at “researchers and students” in philosophy; I’d been impressed and intrigued by a couple of his lucid contributions to Philosophia Mathematica, and had hoped for an equally approachable book. But, whatever Category Theory‘s virtues, easy approachability isn’t one of them, and after reading a fair bit of it, I had to put the book aside for when I had enough time to work through it again more slowly. At last, I’ve got back to it, and I’ll give some reactions here.
I have to say immediately (as in fact I said here before) that I can’t imagine that there are many philosophers who would be equipped to dive straight in and cope with this book. Meeting Cayley’s Theorem (about representing groups as permutations on sets) at p. 11 or free monoids at p. 16 is going to be quite a challenge to those without a background in mathematics. It isn’t that those ideas are intrinsically very difficult; but you surely won’t grasp their point or feel comfortable with the ideas just from their brisk presentations here. Likewise, I bet no one will understand Remark 1.7 (p. 12) on concrete categories who hasn’t already met the idea of “test objects” from elsewhere. By the time the reader gets to the first example of a “universal mapping property” at pp. 17-18, most philosophers surely will be floundering: Awodey’s explanations of what is going on are too terse to help the not-so-mathematical. And things seem only to get worse as the book progresses. I’m pretty sure, then, that this book wouldn’t work as a first introduction to category theory e.g. for philosophy graduate students interested in logic and the philosophy of maths (unless they have an unusually strong background in pure maths already). Although Awodey says in the preface that, if Mac Lane’s book is for mathematicians, his is for ‘everyone else’, in fact Category Theory is actually orientated to students who are, as they say, ‘mathematically mature’.
So, from now on, I’ll be taking the book as in fact operating at (so to speak) a level up from the one Awodey says that it is designed for, i.e. as a follow-up text for mathematically ept readers, to read after mastering e.g Lawvere and Rosebrugh’s Sets for Mathematicians — a follow-up which starts again from scratch to consolidate some basic ideas and then pushes things on deeper and further.
How does the introductory first chapter work on this level? Well, to be frank, still not entirely brilliantly. For example, the whys and wherefores of the first example of a universal mapping property are not really explained that well (nor why we should be particularly interested in free categories). However, on the other side, I like the way that the idea of a functor between categories is introduced early; and some of the illustrative examples of categories and functors between categories in the chapter are illuminating. And the idea of “forgetful functors” comes across nicely.