For later medieval philosophers, writing under the influence of Aristotle’s natural philosophy and metaphysics, the human soul plays two quite different roles, serving as both a substantial form and a mind. To ask the natural question of why we need a soul at all – why we might not instead simply be a body, a material thing – therefore requires considering two very different sets of issues. The first set of issues is metaphysical, and revolves around the central question of (...) why a human being needs a substantial form. The second set of issues is psychological, and turns on the question of why we should suppose that our mind is aptly characterized as a soul. This chapter takes up these two questions in turn, and then turns to whether we should suppose that one and the same thing – a soul – is both substantial form and mind. This dual-function thesis is the most distinctive feature of later medieval psychology, and is one reason that work from this era remains well-worth reading today. Whereas modern thought furnishes many sophisticated discussions of the immateriality of mind, and the metaphysics of body, philosophers since Descartes have rarely considered that it might be one thing, the soul, that accounts for both thought and substantial unity. (shrink)