 Gersonides’ discussion of immortality of the soul in book 1 must be understood against the backdrop of a notoriously difficult passage in Aristotle’s On the Soul, book 3, chapter 5 (430a22–25). In this passage Aristotle seems to postulate the existence of an active intellect which is separable from the passive intellect and which is primarily responsible for the intellectual activities of the human soul. But what is the relation between the active and passive intellects, and which, if either, is immortal? Gersonides states and rejects three positions that elucidate a version of the unity of intellect. The import of Gersonides’ critique of his predecessors can be reduced to three main issues. From a theological perspective, it is clear that the doctrine of unity of intellect threatens the notion of personal immortality. For if all humans share the same intellect, then upon physical death, all that remains of the person is an unindividualized intellect. Epistemologically, the doctrine is unable to account for how it is that two (or more) knowers can entertain contrary items of knowledge; or, more stringently, how one person can be mistaken about something another person knows. And from a metaphysical perspective, the main problem is how to individuate this separate intellect when it is manifested in many individuals: for if it is individuated materially on the basis that individual bodies differ, then the substance is no longer incorporeal or separate. As Feldman has pointed out, on this theory an incorporeal substance is either a unique member of a species or is not a member of a species at all (Wars, I.4, p. 79).