The explicitly psychological works of Siger which are universally accepted as genuine are the Quaestiones in librum tertium de anima, the Tractatus de anima intellectiva, and the De intellectu. Siger’s Quaestiones in librum tertium de anima, which were written sometime between 1265 and 1270, may have been the occasion for Saint Thomas’ De unitate intellectus contra Averroistas, which was issued in 1270. Some of the doctrines contained in them are also to be found in Stephen Tempier’s condemnation of the same (...) year. The distinguished Italian historian of medieval and Renaissance philosophy, the late Bruno Nardi, proved that Siger’s immediate reply to Thomas’ attack was his now lost De intellectu, a work that we know primarily from excerpts and paraphrases to be found in various works of the Renaissance philosopher, Agostino Nifo. While this reply was probably written almost immediately after the appearance of Thomas’ opusculum, that is, in 1270 or 1271, Siger authored yet another reply, the Tractatus de anima intellectiva, which Fernand Van Steenberghen has dated as written in 1272 or 1273. A brief examination of these various works and of Siger’s questions on the Liber de causis, which were written between 1273 and 1276, will make evident the evolution of Siger’s ideas on the soul and the intellect that resulted from his careful study of Saint Thomas’ opusculum. (shrink)