 For excellent comments, thanks are due to Jennifer Hawkins, as well as Dan Brock, Tom Dougherty, Steve Joffe, Neil Manson, Emma Ryman, Robert Truog, David Wendler, Daniel Viehoff, Alan Wertheimer RIP, Dan Wikler, and my students. Work on the 2018 edition was supported by Wellcome 208766/Z/17/Z.