Abstract

In various letters written to Mersenne in 1640 and 1641, Descartes mentions the Summa philosophiae quadrapartita of Eustachius a Sancto Paulo. But nothing that he says would lead us to believe the work an important influence on his thought. Nonetheless, when we turn to the Summa we are immediately struck by what seem to be “Cartesian” elements in its presentation. For example, a distinction is drawn between formal logic and “natural” logic. Formal logic is seen as a convenience in learning the sciences, but natural logic alone is found to be the absolutely essential requirement. And this natural logic is basically a product of the natural light. It is not difficult to see in these passages a foreshadowing of Descartes’ new conception of method. Like Eustachius, he too maintains that the natural functions of the mind are all that we require in the pursuit of truth. But we shall find even more interesting similarities between the two men.