=The Philosophy of the Sophists.= The philosophy of the Sophists was only the logical following out of the general attitude of the time toward all traditions. The more the old physical theories fell into disrepute, the more the changes of the world of politics seemed to indicate instability everywhere, the more opinions differed on the same subject,――so much the more did the possibility present itself to the Sophists of taking two contradictories as equally true, and so much the faster did the whole Greek world lose faith in any valid truth and in any certain knowledge. The dogmatism of the Cosmological Period is thus naturally followed by the skepticism of the Anthropological. Beginning with the cautious and enlightened relativism of Protagoras, there grew up a volume of criticism, until the later Sophists applied destructive doctrines to everything. The best representatives of the philosophical aspect of the Sophistic movement were Protagoras and Gorgias. =1. The Relativism of Protagoras.= Although theoretically skepticism is the centre and logical result of the Sophistic movement, the teaching of the greatest Sophist, Protagoras, cannot be strictly called skepticism. Philosophically, skepticism is not the denial of this or that particular belief as true, but the denial of the existence of any truth whatever. Protagoras refused to make any positive statements――either in denial or affirmation――about ultimate truth, because, as he said, we have no insight whatever into the nature of absolute truth. Our knowledge is confined to motions and the phenomena of motion. His teaching would be called in modern times relativism or phenomenalism. The fundamental principle beneath such a doctrine is that knowledge is human――never absolute, but always relative. The relativism of Protagoras was based on two principles: the first is that of universal change, which he borrowed from Heracleitus; the second is, so far as we know, original with Protagoras,――that sense-perception is the only source and only kind of knowledge. In Heracleitus’ doctrine change is universal, each term of a series of changes passing into another. The senses are a part of this flux, and since they are, according to Protagoras, the only source of knowledge, knowledge is ephemeral and unreal. Reason is extended and continued sensation. A movement external to the organism stimulates an organ of the body and is met by a reacting movement of the organ. The result is perception. Perception being itself a process, each present moment of perception is the only knowledge. We cannot know things as they are in themselves; there is no insight into the Being of things over and above our perceptions. On the contrary, reality is not only what it perceptually appears for each individual, but also what it appears at each individual momentary perception.