The paper discusses the problem of the source of the analogies between philosophical outlook of the Sophists and the skeptical tradition of Pyrrho and his successors. Its main objective is to point out that the similarities in standpoints, arguments and methods between these philosophical phenomena result from the transmission of Plato’s Theaetetus. It is argued that main ideas (phenomenalism, subjectivism, relativity and indeterminacy of things, rejection of being and acceptance of becoming and constant flux, antilogical position consisting in opposing two contradictory statements etc.) which in Theaetetus constituted so called “Secret Doctrine” (attributed by Plato to Protagoras and disciples of Heraclitus), after being criticized by Aristotle in his discussion about the principle of contradiction in Metaphysics (books IV and XI), were acknowledged by Pyrrho and, revived by Aenesidemus, can be found incorporated into the skeptical tradition in the works of Sextus Empiricus.
Keywords: Sophists · Protagoras · Plato · Aristotle · Pyrrho · scepticism · Aenesidemus · Sextus Empiricus
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