Sophists is a title given to the leading public teachers in ancient Greece during the 5th and 4th centuries B.C. The most noted of these were Gorgias of Leontium and Protagoras of Abdera. The foundation of their doctrine was laid in scepticism, absolute truth being denied, and only relative truths being admitted as existing for man. Gorgias attacked the existence of the finite, but at the same time he maintained that all notion of the infinite is unattainable by the human understanding. He expressed his nihilism in three principal propositions: (a) nothing exists; (b) if anything existed, it would be unknowable; (c) if anything existed and were knowable, the knowledge of it could, nevertheless, not be communicated to others. The doctrine of Protagoras was that the phenomena both of external nature and of the processes of mind are so fluctuating and variable that certain knowledge is unattainable. He held that nothing at any time exists, but is always in a state of becoming. Man, he declared, is the measure of all things. Just as each thing appears to each man, so it is for him. All truth is relative. The existence of the gods, even, is uncertain. Thus this leading sophist succeeded in annihilating both existence and knowledge. He founded virtue on a sense of shame and a feeling of justice seated in the human constitution. The sophists made use of their dialectic subtleties as a source of amusement, as well as intellectual exercise, to the youth of Greece. They were opposed by Socrates (q.v.) and Plato, and Aristotle defines a sophist as an imposturous pretender to knowledge — a man who employs what he knows to be fallacy for the purpose of deceit and of getting money." Mr. Grote contends that, so far from this being true, the morality of the Athenian public was greatly improved at the end of the 5th century as compared with the beginning of the century.