The notion of substantial form was central to Aristotelian Scholasticism mainly on two accounts relevant to Descartes. First, natural substances are composites of prime matter and one substantial form (or more forms, depending on the Scholastic). This view was central to the Aristotelian distinction between substantial and accidental change. A substance such as a cow comes into being as a result of a form being “educed from matter,” a natural process. The cow's death consists in the demise of a substance and the separation of matter and form. In the case of “accidental” change (e.g., the cow puts on weight), the substance remains, but its accidents (its qualities, other states not part of its essence) change (Adams 1987, ch. 15). Second, the Scholastics argued that a substance must have a substantial form in which its characteristic qualities and behavior are rooted and united. The form constitutes the nature or essence of the substance, and it explains why humans laugh, horses neigh, and water cools down when removed from a source of heat (Suárez, Disputationes metaphysicae, XV.1). Substantial forms must be distinguished from real qualities, which are accidents.

Descartes held instead that all phenomena in the physical world should be explained mechanistically in terms of qualities that are modes of extension. He often remains studiously quiet about substantial forms, and this was part of a strategy. He thought it imprudent to reject them explicitly (AT VI 239; AT II 199–200, CSMK 107) and assumed that offering his own mechanistic system would automatically lead to an abandonment of substantial forms (AT III 500, CSMK 207). As he explains to Mersenne,