The aim of my paper will be to emphasize the exceptional interest of the Philonian corpus, in order to improve our knowledge of Hellenistic and Middle-Platonic doctrines of gnoseology. We will try to explore the following points: -How to define a methodology for the study of Philonic passages dealing with gnoseology and epistemology? Philo was not a "professional" philosopher; his erudite references are very often mixed, in a very intricate way, with a lot of theological reflections. It would be absurd not to consider such a rich source, but its use requires special care. In any case, the problem of methodology still requires to be deepened. -Philo is a "grand témoin" to the conversion of Neo-Academic scepticism, essentially an anti-Stoic structure, in a doctrine of knowledge founded on Platonic concepts. At the same time, he is the first to have heard about the Neo-Pyrrhonian tropes, coined by Aenesidemus. The powerful dynamic of the evolution and confrontation of doctrines is intensely present in his work. We will try to demonstrate that the concepts of "eclecticism" and "syncretism" are particularly inappropriate to account for this presence. -How are these philosophical concepts integrated in the negative theology that is the entire reason for Philo's writing his exegetical treatises?