 That There are Incorporeal Substances (Abu Rida 1950, 265–69, Adamson and Pormann 2009) is a creative application of ideas from Aristotle’s Categories to the problem of showing that the human soul is an immaterial substance. Al-Kindi takes up this task in stages, first proving that the soul is a substance, then showing that it is immaterial. He argues that the soul is a substance by drawing on the opening chapters of the Categories to claim that the essence of something shares a name and definition with that thing. Since the soul is the essence of the living being, and the living being is a substance, the soul is also a substance. Furthermore, it is an immaterial substance: for the soul is “the intellectual form of the living thing,” and an intellectual form is a species. But species, al-Kindi argues, are immaterial; therefore the soul is immaterial. Among the problematic moves in this train of argument is the identification of the human soul with the species of human. This would seem to be al-Kindi’s attempt to bring together the idea of species, which is a “secondary substance” in the Categories, with the doctrine of form found in such works as the De Anima and Metaphysics. Al-Kindi simply conflates the two, without argument – he does not address the obvious question of how there can be many human souls, all of which are identical with the single species human.