Descartes’ ingenium, corresponding to French esprit and usually translated “mind,” is more accurately rendered as “native intelligence” or “native wit.” It occurs in the correspondence with Isaac Beckman in 1619 and chiefly and frequently in the unfinished Rules for the Direction of the Mind, which begins with the statement that “the aim of studies ought to be the direction of ingenium so that it brings forth solid and true judgments about all things that occur” (AT X 359, CSM I 9). This first Cartesian treatment of method is an account of ingenium’s proper employment.

Ancient and Renaissance rhetoric regarded ingenium as a disposition essential to oratorical and poetic invention. Early medieval Scholastics considered it, alongside memory, as necessary for intensive study and meditation. The early seventeenth-century philosophical lexicographer Rudolphus Goclenius wrote that this “power of successfully and easily discovering and contriving in human beings” depended on both body and mind, on “the constitution of the organs, and on the auxiliary faculties, like the phantasia” (Sepper 1996, 90).

Descartes’ Rules is indebted to this psychophysiological tradition. The chief aids to intellect are common sensation (see common sense), memory, and imagination. They are the various ways in which the knowing power (vis cognoscens) acts in and through the organ of phantasia (which eventually becomes the “little gland,” the pineal gland, of the Treatise on Man). When properly regulated, they enable one to intuit simple things distinctly, to combine complexes correctly, and to compare all things human powers can access (see simple nature). Rule 12 defines ingenium as the knowing power insofar as it “forms new ideas in the phantasia, or concentrates on those already formed” (AT X 416, CSM I 42). Since “idea” in the Rules is used for corporeal and mathematical images, ingenium amounts to the active, inventive power of forming and concentrating on images. It is the ultimate source of Descartes’ analytic geometry, which dynamically generates new figures from existing ones by imaginative manipulations that are tracked and anticipated using algebraic equations. Although the later Descartes’ interest in the term ingenium fades (it is partially absorbed into concepts like bona mens and esprit), the essence of its practice, process, and regulation lives on in his concern with method, especially mathematical method.