The classic Arabic bibliographies ascribe to al-Fārābī a treatise entitled Fī māhiyyat al-nafs (“On the Essence of the Soul”), of which no Arabic manuscript is known to exist. There is however a Hebrew text, translated from the Arabic by Zeraḥiah ben She'altiel Ḥen of Rome in 1284, which is ascribed to al-Fārābī in all the manuscripts and which carries the title Ma'amar be-mahut ha-nefesh (“Treatise on the Essence of the Soul”). Since Steinschneider, this text is taken to be the translation of al-Fārābī's treatise lost in the original. In this paper I argue that the Ma'amar be-mahut ha-nefesh is not by al-Fārābī: it is in vain that one looks in it for ideas characteristic to the Second Master, while the ideas expressed therein are incompatible with those of al-Fārābī. I offer a study of Ma'amar be-mahut ha-nefesh, followed by an annotated translation into French. This is a “popular” Neoplatonic text, whose ideas are mostly very ordinary, but in part uncommon. It notably explains the emergence of forms in matter by positing the existence of a pneuma that by circulating around the body “brings forth” the vegetative and animal souls. The author also draws on an unusual notion called hemshel in Hebrew (a term probably translating the Arabic mithāl, rendered as exemplum in Latin): the exemplum is said to become visible as a result of the pneuma's circular motion. The paper is followed by a short Note by Rémi Brague looking into the sources of the treatise. Brague concludes that the text cannot be assigned to any specific tradition.