The present volume takes its start from a set of six lectures delivered to the Department of Philosophy of Renmin University in Beijing during the last two weeks of October 2016, as the eighth in an ongoing series of Master Classes in Ancient Greek Philosophy. I am most grateful to Professor Wei Liu of the Department for the original invitation, and for copious hospitality during my stay. A version of the lectures is due to be published in due course in Chinese translation. The original lectures have been considerably revised for the present edition, and I am greatly indebted to Michael Sharp of Cambridge University Press for accepting my proposal to turn a set of somewhat disconnected papers into a book, and to the two anonymous readers that he selected for many useful suggestions as to achieving that goal. The remaining inadequacies are my own.