In the general treatment of the subject, I have followed in the main the order, and drawn chiefly on the selection of passages, in Ritter and Preller's Historia Philosophiae Graecae. It is hoped that in this way the little book may be found useful at the universities, as a running commentary on that excellent work; and the better to aid students in the use of it for that purpose, the corresponding sections in Ritter and Preller are indicated by the figures in the margin. In the sections on Plato, and occasionally elsewhere, I have drawn to some extent, by the kind permission of the Delegates of the Clarendon Press and his own, on Professor Jowett's great commentary and translation.