This isn’t actually the title I gave for the talk at the time, and this isn’t the first time I gave the talk — I had given previous versions of it at Oxford and Aberystwyth over the past week too. but I think this is the best version, and I also think that this title captures much better what I am actually talking about. So it will probably be the title of the article or chapter I end up writing. Eventually. In the meantime, here are some slides, and here’s the audio of the talk.