2. TREATMENT OF KNOWLEDGE IN THE HISTORICAL SYSTEMS OF PHILOSOPHY A. In Greek Philosophy.—The problems which are now grouped under epistemology are many of them nearly as old as philosophy itself. It is true that human reflection always looks outward before it looks inward; and that, consequently, cosmo logical inquiries into the nature of the physical world precede in the order of time inquiries into the way in which knowledge of the physical world is acquired. In Greek thought, these cos mological inquiries centred round the attempt to discover a " mate rial principle" or "cause" of things. From the time of Thales (born about 625 B.c.) to that of Protagoras (born about 480 B.c.) there can be traced a tolerably well demarcated line of speculation the aim of which was to come upon some known permanent substra tum of natural fact and to exhibit the ways in which that sub stratum is connected with the particular phenomena of concrete experience. Only by degrees did problems of knowledge come into prominence, and therefore, only gradually did there evince itself a deliberate trend of human reflection towards their solu tion. Probably Socrates (born about 470 B.c.) first set the cur rent of Greek thought definitely in this direction. The view that Socrates was a mere moralist is ill-founded.
That Socrates did select practical conduct as matter worthy above all of rational consideration can scarcely be disputed. And to do this was in effect to accentuate the intimate relation of ob jective fact to human thought. It was as introducing this subjec tive element into Greek speculation that Socrates brought to the front issues of an epistemological character. Doubtless he was crystallizing much of the current opinion of his time—particu larly that which prevailed among the Sophists and their followers. —but no sooner had the step been taken than we find Greek philosophy occupied with special and well-marked problems of the kind we should now call epistemological.
In the Theaetetus (completed about 368 B.c.), three theories of
the nature of knowledge (irto-riym) are submitted to examination. In this dialogue Plato's aim is to show that these three theories break down under the weight of criticism that may be brought to bear upon them.
More than half the dialogue is occupied with a theory which was not only that propounded by certain well-known philosophic inquirers but an answer to the question likely to be given by any one who has pondered over the matter,—namely, that knowledge is identical with sense-apprehension This view is im mediately connected with the doctrine of Protagoras that "man is the measure of all things," and interpreted as implying the rela tivist position that things are for me as they appear to me, and for you as they appear to you. A thing is to each man as it ap pears to him, and to say that it appears to him is equivalent to saying that he perceives it. Sensation, then, is of something that is, and is exempt from error; it is certain knowledge. As Taylor has pointed out, it is not a doctrine of "subjectivism" that is here attributed to Protagoras ; it is not suggested that he meant to assert of "what appears" that it is a mental state of the percipient.
Rather the point is that, according to Protagoras, we are not en titled to assume a common real world apprehensible by any num ber of percipients; each individual percipient lives in a private world known only to himself, and, since no two of these private worlds have anything in common, each percipient can be said to be infallible in regard to what he perceives. Socrates is repre sented as attempting to work out the theory more elaborately be fore proceeding to judge its merits. It virtually depends, he argues, upon an ontological position, such as that which had been advocated by Heracleitus, that every thing real is in a condition of ceaseless flux, that what is called "being" is but change. So re garded the entities we speak of as "existents" are streams of events and movement is the ultimate reality.