During classical Antiquity, Stoïcists seem to have thought the most radically on the difference between man and animal. Their thesis, hardly criticized by the Academicians, are based on the affirmation that animals are incapable of "internal discurse", though some of them may have vocal expression. This theory is surprising as far as the condition of man's oikeiôsis is concerned. This paper is dealing with two texts: Seneca's letter 121 and the beginning of a treatease on Ethic of Hierocles, transmissed by Pap. berol. 9780 v. Indeed these texts give a peculiar presentation on importance and limits of "perception of oneself" (proprioception) of which animals are capable.