In the spring of 1966, Gregory Vlastos invited me (among others) to submit a paper for consideration for an issue of the Monist he was editing. I did. Gregory did not accept the paper, but generously (as was his way always) provided me with detailed comments. One of those comments was: “To do this, you'd have to write a book.” Here it is.

Along the way I have incurred an enormous number of intellectual debts. There is no possibility of my thanking all of those who have helped; for one thing, I would have to include all the students in seminars who have asked penetrating questions and made perceptive comments. So I'll confine myself to the oldest debt, that owed to Gregory Vlastos, and the most recent ones.

Michael Ruse sat through the better part of a seminar I gave on the book in 2002–2003, was obstreperous (often helpfully), and got me to submit the manuscript to Cambridge University Press. I won't say that without his prodding the book would never have got out, but I won't deny it either.

Hilary Gaskin, of Cambridge University Press, also pushed me to submit it. She has been unfailingly encouraging throughout the entire process.

She had it sent on to two referees, who gave me further reason to push on. One of them, Eric Brown, gave me very extensive comments indeed.