The doctrine of the substantial form is itself a Scholastic innovation. Nowhere does Aristotle employ this term, and though Aquinas and others take him to imply the existence of such a form in certain passages of the Physics and Metaphysics, it is not clear that Aristotle was committed to such a form in the Scholastic sense. Even if one agrees with Scholastics that Aristotle was committed to substantial forms, the Christian doctrine of the immortality of the soul required significant revisions to the sense in which Aristotle took the soul to be the form of the body. In this chapter, I will first examine the main argument Aquinas presents for the distinction between substantial and accidental forms in his commentaries on Aristotle's Physics and Metaphysics. Then, as background to Suarez's alternative argument for the substantial form, I will highlight certain tensions created by Aquinas' attempt to account for the immortality of the soul in these terms in the Summa Theologica. I will conclude that, despite Aquinas' status as the theologian all Jesuits were to follow on non-controversial issues, and the fact that Descartes owned a copy of the Summa Theologica, Aquinas' view is not the target of Descartes' a priori metaphysical argument.

Since my goal is to provide background to Suarez's view of the substantial form and make sense of Descartes' arguments against material substantial forms, I will limit myself to Aquinas' account of the forms found in Aristotelian natural philosophy.