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. Wherever and whenever the authority of ancient traditions is broken down, the doctrine that one man’s opinion is as good as another’s immediately takes its place; or rather the doctrine in question is a survival of traditionalism in an extremely pulverised form. And when we are told that the majority must be right—which is a very different principle from holding that the majority should be obeyed—we may take it as a sign that the loose particles are beginning to coalesce again. The substitution of an individual for a universal standard of truth is, according to Plato, a direct consequence of the theory which identifies knowledge with sense-perception. It is, at any rate, certain that the most vehement assertors of the former doctrine are also those who are fondest of appealing to what they and their friends have seen, heard, or felt; and the more educated among them place enormous confidence in statistics. They are also fond of repeating the adage that an ounce of fact is worth a ton of theory, without considering that theory alone can furnish the balance in which facts are weighed. Plato does not go very deep into the
rationale of observation, nor in the infancy of exact science was it to be expected that he should. He fully recognised the presence of two factors, an objective and a subjective, in every sensation, but lost his hold on the true method in attempting to trace a like dualism through the whole of consciousness. Where we should distinguish between the mental energies and the physical processes underlying them, or between the elements respectively contributed to every cognition by immediate experience and reflection, he conceived the inner and outer worlds as two analogous series related to one another as an image to its original.
At this last point we touch on the final generalisation by which Plato extended the dialectic method to all existence, and readmitted into philosophy the earlier speculations provisionally excluded from it by Socrates. The cross-examining elenchus, at first applied only to individuals, had been turned with destructive effect on every class, every institution, and every polity, until the whole of human life was made to appear one mass of self-contradiction, instability, and illusion. It had been held by some that the order of nature offered a contrast and a correction to this bewildering chaos. Plato, on the other hand, sought to show that the ignorance and evil prevalent among men were only a part of the imperfection necessarily belonging to derivative existence of every kind. For this purpose the philosophy of Heracleitus proved a welcome auxiliary. The pupil of Socrates had been taught in early youth by Cratylus, an adherent of the Ephesian school, that movement, relativity, and the conjunction of opposites are the very conditions under which Nature works. We may conjecture that Plato did not at first detect any resemblance between the Heracleitean flux and the mental bewilderment produced or brought to light by the master of cross-examination. But his visit to Italy would probably enable him to take a new view of the Ionian speculations, by bringing him into contact with schools maintaining a directly opposite doctrine. The Eleatics held that existence remained eternally undivided, unmoved, and unchanged. The Pythagoreans arranged all things according to a strained and rigid antithetical construction. Then came the identifying flash.132
