Abstract
Motion and relation are key themes in later Platonic philosophy. The article examines the role of these themes in Protagoras''s "secret doctrine" in Plato''s Theaetetus.
This doctrine is based on a combination of specific relationality (namely the "bipolar" subjective relationality whose model is perception) and a specific motion (namely the motion of generation and corruption) and forms an "ontological basis" for the epistemological-ethical conglomerate of Protagoras'' sophistic rhetoric. Plato in his later philosophy oppose this conglomerate with an alternative ontology founded on a different and wider conception of relation and motion and in this way incorporates some modified elements of this "secret doctrine" and rhetoric in his own philosophy.