We need not follow Plato’s investigations into the meaning of knowledge and the causes of illusion any further; especially as they do not lead, in this instance, to any positive conclusion. The general tendency is to seek for truth within rather than without; and to connect error partly with the disturbing influence of sense-impressions on the higher mental faculties, partly with the inherent confusion and instability of the phenomena whence those impressions are derived. Our principal concern here is to note the expansive power of generalisation which was carrying philosophy back again from man to Nature—the deep-seated contempt of Plato for public opinion—and the incipient differentiation of demonstrated from empirical truth.
A somewhat similar vein of reflection is worked out in theCratylus, a Dialogue presenting some important points of contact with theTheaetêtus, and probably belonging to the same period. There is the same constant reference to Heracleitus, whose philosophy is here also treated as in great measure, but not entirely, true; and the opposing system of Parmenides is again mentioned, though much more briefly, as a valuable set-off against its extravagances. TheCratylusdeals exclusively with language, just as theTheaetêtushad dealt with sensation and mental imagery, but in such a playful and ironical tone that its speculative importance is likely to be overlooked. Some of the Greek philosophers seem to have thought that the study of things might advantageously be replaced by the study of words, which were supposed to have a natural and necessary connexion with their accepted meanings. This view was particularly favoured by the Heracleiteans, who found, or fancied that they found, a confirmation of their master’s teaching in etymology. Plato professes to adopt the theory in question, and supports it with a number of derivations which to us seem ludicrously absurd, but which may possibly have been transcribed from the pages of contemporary philologists. At last, however, he turns round and shows that other verbal arguments, equally good, might be adduced on behalf of Parmenides. But the most valuable part of the discussion is a protest against the whole theory that things can be studied through their names. Plato justly observes that an image, to be perfect, should not reproduce its original, but only certain aspects of it; that the framers of language were not infallible; and that we are just as competent to discover the nature of things as they could be. One can imagine the delight with which he would have welcomed the modern discovery that sensations, too, are a language; and that the associated groups into which they most readily gather are determined less by the necessary connexions of things in themselves than by the exigencies of self-preservation and reproduction in sentient beings.
Through all his criticisms on the popular sources of information—sense, language and public opinion—Plato refers to an ideal of perfect knowledge which he assumes without being able to define it. It must satisfy the negative condition of being free from self-contradiction, but further than this we cannot go. Yet, in the hands of a metaphysician, no more than this was required to reconstruct the world. The demand for consistency explains the practical philosophy of Socrates. It also explains, under another form, the philosophy, both practical and speculative, of his disciple. Identity and the correlative of identity, difference, gradually came to cover with their manifold combinations all knowledge, all life, and all existence.
It was from mathematical science that the light of certainty first broke. Socrates had not encouraged the study of mathematics, either pure or applied; nor, if we may judge from some disparaging allusions to Hippias and his lectures in theProtagoras, did Plato at first regard it with any particular favour. He may have acquired some notions of arithmetic and geometry at school; but the intimate acquaintance with, and deep interest in them, manifested throughout his later works, probably dates from his visits to Italy, Sicily, Cyrênê, and Egypt. In each of these places the exact sciences were cultivated with more assiduity than at Athens; in southern Italy they had been brought into close connexion with philosophy by a system of mystical interpretation. The glory of discovering their true speculative significance was reserved for Plato. Just as he had detected a profound analogy between the Socratic scepticism and the Heracleitean flux, so also, by another vivid intuition, he saw in the definitions and demonstrations of geometry a type of true reasoning, a particular application of the Socratic logic. Thus the two studies were brought into fruitful reaction, the one gaining a wider applicability, and the other an exacter method of proof. The mathematical spirit ultimately provedtoo strong for Plato, and petrified his philosophy into a lifeless formalism; but no extraneous influence helped so much to bring about the complete maturity of his constructive powers, in no direction has he more profoundly influenced the thought of later ages.
Both theTheaetêtusand theCratyluscontain allusions to mathematical reasoning, but its full significance is first exhibited in theMeno. Here the old question, whether virtue can be taught, is again raised, to be discussed from an entirely new point of view, and resolved into the more general question, Can anything be taught? The answer is, Yes and No. You may stimulate the native activity of the intellect, but you cannot create it. Take a totally uneducated man, and, under proper guidance, he shall discover the truths of geometry for himself, by virtue of their self-evident clearness. Being independent of any traceable experience, the elementary principles of this science, of all science, must have been acquired in some antenatal period, or rather they were never acquired at all, they belong to the very nature of the soul herself. The doctrine here unfolded had a great future before it; and it has never, perhaps, been discussed with so much eagerness as during the last half-century among ourselves. The masters of English thought have placed the issue first raised by Plato in the very front of philosophical controversy; and the general public have been brought to feel that their dearest interests hang on its decision. The subject has, however, lost much of its adventitious interest to those who know that theà prioriposition was turned, a hundred years ago, by Kant. The philosopher of Königsberg showed that, granting knowledge to be composed of two elements, mind adds nothing to outward experience but its own forms, the system of connexions according to which it groups phenomena. Deprive these forms of the content given to them by feeling, and the soul will be left beating her wings in a vacuum. The doctrine that knowledge is not adead deposit in consciousness or memory, but a living energy whereby phenomena are, to use Kant’s words, gathered up into the synthetic unity of apperception, has since found a physiological basis in the theory of central innervation. And the experiential school of psychology have simultaneously come to recognise the existence of fixed conditions under which consciousness works and grows, and which, in the last analysis, resolve themselves into the apprehension of resemblance, difference, coexistence, and succession. The most complex cognition involves no more than these four categories; and it is probable that they all co-operate in the most elementary perception.
The truths here touched on seem to have been dimly present to the mind of Plato. He never doubts that all knowledge must, in some way or other, be derived from experience; and, accordingly, he assumes that what cannot have been learned in this world was learned in another. But he does not (in theMenoat least) suppose that the process ever had a beginning. It would seem that he is trying to express in figurative language the distinction, lost almost as soon as found, between intelligence and the facts on which intelligence is exercised, An examination of the steps by which Meno’s slave is brought to perceive, without being directly told, the truth of the Pythagorean theorem, will show that his share in the demonstration is limited to the intuition of certain numerical equalities and inequalities. Now, to Plato, the perception of sameness and difference meant everything. He would have denied that the sensible world presented examples of these relations in their ideal absoluteness and purity. In tracing back their apprehension to the self-reflection of the soul, the consciousness of personal identity, he would not have transgressed the limits of a legitimate enquiry. But self-consciousness involved a possible abstraction from disturbing influences, which he interpreted as a real separation between mind and matter; and, to make it more complete, an independent pre-existence of the former. Nor was this all. Since knowledge is of likeness in difference, then the central truth of things, the reality underlying all appearance, must be an abiding identity recognised by the soul through her previous communion with it in a purer world. The inevitable tendency of two identities, one subjective and the other objective, was to coalesce in an absolute unity where all distinctions of time and space would have disappeared, carrying the whole mythical machinery along with them; and Plato’s logic is always hovering on the verge of such a consummation without being able fully to accept it. Still, the mystical tendency, which it was reserved for Plotinus to carry out in its entirety, is always present, though restrained by other motives, working for the ascertainment of uniformity in theory and for the enforcement of uniformity in practice.
We have accompanied Plato to a point where he begins to see his way towards a radical reconstruction of all existing beliefs and institutions. In the next chapter we shall attempt to show how far he succeeded in this great purpose, how much, in his positive contributions to thought is of permanent, and how much of merely biographical or literary value.