The article, after providing a general survey of prescriptive theories (or theories on rules) in Hellenistic philosophy, tries to offer a detailed analysis of the moral doxography of Philo of Larissa conserved in Stobaeus' Anthology (Stob. Ecl. II 7, 2, pp. 39-40 W.-H. = 25 Wiśniewski, 2 Mette, 32 Brittain). According to this evidence, Philo divided moral philosophy in three parts : hortatory, or protreptic, topos, therapeutic topos, prescriptive topos –; besides, he parted the prescriptive topos into general and particular logos. This doctrine is contextualised in rhetoric reflection of Hermagorean school and also considered in the light of Academic probabilism. In particular, Carneades' theory of the representations attested by Sextus, M VII 184 is the background against which to evaluate Philo's position.