2006

Greg Restall has put a very nice paper online, called ‘Anti-realist classical logic and realist mathematics’. I’ve always been tempted by logicism in the very broadest sense: but Greg’s critics, of course, will say that he’s just smuggled the rabbit into the hat before pulling it out again. Still, his piece is nicely thought-provoking.

Oddly, given his multiple-conclusion logical framework, Greg doesn’t mention Shoesmith and Smiley’s great book in his biblio. (Very regrettably, it had the bad luck to be published in 1978, around when a number of publishers used early computers to print books with what looked like typewritten pages. The first edition of Fogelin’s fine book on Wittgenstein suffered the same fate. And in both cases, I think the repellent and amateurish look of the results was enough to put readers off and stop the books making the impact they deserved at the time. At least Fogelin got a properly printed second edition. But Shoesmith and Smiley has gone out of print, and seems widely forgotten.)

Kai von Fintel has just e-mailed, to send a link to the Good Math, Bad Math blog on category theory which is excellent — a series of mini-essays on concepts of category theory with some very helpful introductory explanations of some of the Big Ideas. Worth checking out, so thanks Kai!

I’ve now read quite a lot of Steve Awodey’s new book. Disappointing: or at least, it doesn’t do quite what I was hoping it would do. Awodey’s two papers in Philosophia Mathematica were among the pieces that got me interested in category theory in the first place (see his ‘Structure in mathematics and logic: a categorical perspective’, 1996, and his reply to Hellman, 2004). So I suppose I was hoping for a book that had more of the discursive, explanatory, commentary that Awodey is good at. But there’s very little of that. And 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 still pretty well orientated to maths students (for example, there is a telegraphic proof sketch of Cayley’s Theorem that every group is isomorphic to a permutation group by pp. 11-12!). Of course, there are good things: for just one example it helped me understand better the general idea of limits and colimits. But I wouldn’t really recommend this book to the non-mathematician.

So over the next week or two, I think it’s Goldblatt’s lucid Topoi, and Lawvere and Rosebrugh’s Sets for Mathematics for me.

A copy of Steve Awodey’s new Category Theory (Oxford Logic Guides 49) has just been delivered. His preface starts

Why write a new textbook on Category Theory, when we already have Mac Lane’s Categories for the Working Mathematician? Simply put, because Mac Lane’s book is for the working (and aspiring) mathematician. What is needed now … is a book for everyone else.

First impressions look very encouraging, though the book covers quite a lot in under 250 pages, judging from the table of contents, so the pace promises to be pretty speedy. (Grumble to OUP: given this is intended to be a textbook for a relatively wide audience, why exactly is it published only in hardback and at the extremely steep price of £65 pounds/$124.50?)

I’ve just noticed too that Robert Goldblatt’s Topoi: The Categorical Analysis of Logic — which has been out of print for ages — has been republished by Dover (Amazon have it for £14.63/$19.77 which is a bargain for a book twice the length of Awodey’s). I found before that Goldblatt’s book starts pretty gently in a very helpful way, even though it seems to accelerate a bit alarmingly after the first three chapters. Anyway, my plan of action now is to parallel-process Goldblatt, Awodey, and Lawvere/Rosebrugh. Watch this space for further reports!

(Mmm, I hope that one or two greedy booksellers on abebooks.com who have been trying to sell on second-hand copies of Topoi for quite extortionate prices up to $500 have just found themselves stuck with now unwanted stock!)

I said I’d post occasional progress reports on getting to grips with Category Theory, to pass on recommendations about what I’ve found helpful, etc. (Perhaps I should explain that — when trying to get into a new chunk of maths or math. logic — I find it works best to dive in and read a number of books quite quickly, skipping and skimming when the mood takes me, rather than plod very carefully through one key text. Also, though I know a few bits and pieces, I thought I should start over again from the very beginning.)

Two frequent recommendations for entry points are Lawvere and Schanuel’s Conceptual Mathematics (CUP, 1997), and Lawvere and Rosebrugh’s Sets for Mathematicians (CUP, 2003). But I can only say, give the first book a miss. It’s not very good: the authors seem to have no clear idea about their intended audience, so they veer between the irritatingly condescending/extremely laboured on the one hand, and sudden jumps in difficulty/sheer opaqueness on the other. For just one example, it is very difficult to believe that someone who needs a noddy explanation of why proving if p, q establishes if not-q, not-p is going to make anything at all of the sudden excursus on Gödel and Tarski at p. 307. But worse, you can get to the end of the book with no clear idea about what it is supposed to have achieved, or why it might matter. The second book is so far proving also a bit uneven but much better.

I’m dipping into various expository/philosophical articles as I go along. One I’ve just come across which I found prokoving and illuminating is Barry Mazur’s ‘When is one thing equal to some other thing?‘. Recommended.

I’m still half-buried under tripos marking, but the end is in sight. And I’m Chair of the Examining Board for Parts IB and II this year, which is also not exactly an anxiety-free job. But between times, I’m trying to reorganize and finish the chapters of my book on the Second Theorem. I’m stuck for the moment wondering what exactly to say about “the best and most general version of the unprovability of consistency in the same system” which Gödel so briefly alludes to in the first part of his 1972a note (which repeats a footnote from 1967). Feferman in his editor’s introduction explains things by bringing to bear Jeroslow’s 1973 result. But it isn’t entirely clear to me that this rather esoteric result must be what Gödel had in mind.

Meanwhile, a nerdy footnote. There’s an even better new version out of NoteBook just out (if you are a Mac user, it really is just indispensable, and the academic price is absurdly low).