by an analogous inspiration to the idealising soul of Plato. Each had learned to practise, although for far different purposes, the royal art of Socrates—the mastery over men’s minds acquired by a close study of their interests, passions, and beliefs. But the ambition of the one defeated his own aim, to the destruction of his country and of himself; while the other drew into Athenian thought whatever of Western force and fervour was needed for the accomplishment of its imperial task. We may say of Plato what he has said of his own Theaetêtus, that ‘he moves surely and smoothly and successfully in the path of knowledge and inquiry; always making progress like the noiseless flow of a river of oil’;[131] but everywhere beside or beneath that placid lubricating flow we may trace the action of another current, where still sparkles, fresh and clear as at first, the fiery Sicilian wine. It will be remembered that in an earlier section of this chapter we accompanied Plato to a period when he had provisionally adopted a theory in which the Protagorean contention that virtue can be taught was confirmed and explained by the Socratic contention that virtue is knowledge; while this knowledge again was interpreted in the sense of a hedonistic calculus, a prevision and comparison of the pleasures and pains consequent on our actions. We have now to trace the lines of thought by which he was guided to a different conception of ethical science. After resolving virtue into knowledge of pleasure, the next questions which would present themselves to so keen a thinker were obviously, What is knowledge? and What is pleasure? The _Theaetêtus_ is chiefly occupied with a discussion of the various answers already given to the first of these enquiries. It seems, therefore, to come naturally next after the _Protagoras_; and our conjecture receives a further confirmation when we find that here also a large place is given to the opinions of the Sophist after whom that dialogue is named; the chief difference being that the points selected for controversy are of a speculative rather than of a practical character. There is, however, a close connexion between the argument by which Protagoras had endeavoured to prove that all mankind are teachers of virtue, and his more general principle that man is the measure of all things. And perhaps it was the more obvious difficulties attending the latter view which led Plato, after some hesitation, to reject the former along with it. In an earlier chapter we gave some reasons for believing that Protagoras did not erect every individual into an arbiter of truth in the sweeping sense afterwards put upon his words. He was probably opposing a human to a theological or a naturalistic standard. Nevertheless, it does not follow that Plato was fighting with a shadow when he pressed the Protagorean dictum to its most literal interpretation. There are plenty of people still who would maintain it to that extent.