The article seeks to trace the image Plato has of Heraclitus and connect it with the argumentative structure of the Cratylus in order to understand the textual needs that give rise to the doctrine of perpetual flux, that is, the discussion regarding the correctness (ὀρθότης) of names. The inclusion of Heraclitus's testimony makes it possible to trace the alleged consolidation of the thesis regarding primary and sec - ondary names as the axis of separation between two levels of reality (one stable, the other, changing) and the theory of Ideas -that is, as the basis of Plato's epistemol - ogy as set forth in the late dialogues-.