The term “Cartesian dualism” refers to Descartes’ claim that mind and body are “really distinct”substances. Following some brief remarks on the background to this claim, this entry examines Descartes’ arguments for the claim and its content (i.e., what exactly “mind” and “body” encompass for Descartes). Further related issues (mind-body interaction, the “substantial union” of mind and body) are treated elsewhere in this volume (see human being).
Descartes’ Aristotelian predecessors held that souls were the principle of life and hence that all living things had souls. Living things, and hence souls, formed a hierarchy: plants possessed “vegetative” souls, which performed the functions of nutrition and reproduction; nonhuman animals possessed “sensitive” souls further responsible for locomotion, sensation, and imagination; human beings alone possessed “rational” souls, which were responsible for thinking, judging, and willing (see, e.g., Eustachius 1609, pt. III.iii). Descartes’ dualism amounted to a radical overhaul of this tradition. On the one hand, in identifying the essence of body as extension and in insisting that everything corporeal was mechanically explicable, he brought life, and the other functions of the vegetative and sensitive souls, within the scope of mechanical explanation: the difference between life and death is analogous to that between a clock that is functioning properly and one that is broken (AT XI 330–31, CSM I 329–30). On the other hand, he eliminated the need for any soul other than the rational soul (AT XI 202, CSM I 108), which in his hands became the mind: an immaterial thinking substance; it alone was outside the scope of mechanical explanation (see Baker and Morris 1996, 69ff., and Rozemond 2006).
1. Descartes’ Argument for Mind-Body Dualism
We focus here on Descartes’ main argument for the claim that mind and body are distinct substances (AT VII 78, CSM II 54). Space precludes treatment of a second argument, grounded in the ideas that “the body is by its very nature always divisible, while the mind is utterly indivisible” (AT VII 85–86, CSM II 59) (see divisibility).
Descartes’ argument cannot be fully understood without some prior grasp of his linked conceptions of substance, essence (or principal attribute), and (real) distinction (all of these terms were common currency among his Scholastic predecessors and contemporaries, although individual conceptions varied). His clearest explanations of these terms occur in the Principles.