  Thanks to audiences at the Sigma Group at the LSE, Leeds HPS, and the first year seminar in Oxford, for comments on presentations of parts of this material, and to Alan Hájek, Chris Porter, and Fred Kroon for extensive and very helpful comments on a draft entry. (In particular, the argument of supplement A.2, note 4 is due to Hájek.) In revising the entry, I’ve been grateful to Chris Porter for further helpful comments and pointers to some of the recent literature. The ‘pluralist’ approach mentioned in supplement  B.1.2 is due to him, as in the broad outlines of the argument of  B.1.3.