## One True Logic

There’s a new book out by Owen Griffiths and Alex Paseau, *One True Logic: A Monist Manifesto* (OUP). As the title suggests, this argues against logical pluralism. Yes, of course, there are myriad logical systems which we can concoct and study as interesting mathematical objects. But what about the logic we actually use in reasoning *about* them and about other mathematical matters? Is there in fact one correct logic which tracks what really does follow from what? Our authors take a conservative line, in that they are anti-pluralist: there is indeed one true logic for in-earnest applications. They are unconservative in defending a highly infinitary logic in that role.

I’ve read the first few chapters with enjoyment and enlightenment. But I’m going to have to shelve the book for the moment, as it will be too distracting from other commitments to engage seriously with the rest of it for a while. One of the delights of somewhat senior years is finding it more difficult to think about more than one thing at a time. (“But what’s new?” murmurs Mrs Logic Matters from the wings.)

For a start, I must continue cracking on with the category theory project. I have now revised Chapters 1 to 15 of *Beginning Category Theory*. So here they are, in one long PDF which also includes the remaining unrevised chapters from the 2015/2018 *Gentle Intro*.

In this iteration there are quite a few minor changes to Chapters 1 to 13 (correcting typos, clarifying some phrasing, deleting an unnecessary section, adding a new theorem in §12.2, etc.), though there is nothing very significant there. I have also now revised Chapter 15, the first of the two general chapters on limits/colimits. This and the preceding chapter on equalisers/co-equalisers could surely do with more polishing and lightening-up in places. But as I’ve said previously, I’m including revised chapters when they are at least an improvement on what went before (I’m not waiting for final-draft perfection!).

If you are like me, you are looking for the more-than-occasional consoling distractions from the state of the wider world. Let me share one.

Of the great pianists I have had the chance to hear live over the years, the one I perhaps find the most emotionally engaging of all is Maria João Pires. Her unmannered directedness goes straight to the heart. So here she is, playing Schubert, Debussy and Beethoven, in a video recorded in Gstaad last August.