Descartes’ Philosophy: Doubt, Certainty, and the Mind-Body Problem
Doubt and Certainty
In Descartes’s philosophy, doubt is the starting point. Convinced that everything must undergo a critical review, he begins to doubt everything (as presented in Part 3 of Discourse on the Method). Cartesian doubt differs from skepticism because it is methodical and preliminary: one must hesitate before reaching certainty, and doubt destroys itself to generate certainty. First, truths susceptible to the senses are rapidly dismissed (rejecting the rationalism characteristic of sensitive knowledge). Then, the question arises about the existence of external reality. Descartes introduces the sleep hypothesis (fictitious or hypothetical), after which we cannot be certain of being awake or not. Still, mathematical truths remain valid. Doubt becomes ultimate (hyperbolic) with the hypothesis of the evil genius (deus trickster), where a superior being might continuously deceive us, even in mathematical matters. Thus, all certainty seems lost. However, if I am mistaken, I must exist to be deceived; when I think I am mistaken, I am existing. Therefore, if I think, I am (cogito ergo sum). This statement is the first certainty, withstanding all tests of doubt. Descartes takes the cogito as the first self-evident truth, clearly and distinctly understood (a clear and distinct idea in the mind, separate and well-defined), and therefore, undoubtedly true. Descartes believed that things recognized so clearly and distinctly would be true, i.e., those taken as obvious. The cogito is not an application of the rule of evidence, reasoning, or deduction from a previous assumption. It involves simultaneity: thinking and existing are not sequentially related, but while I find that I exist, it may well be that if I stopped thinking, I would also cease to exist. The cogito is the existential self-evidence, reached after undergoing the most exaggerated (hyperbolic) doubt. Admitting to thinking implies existing, an intellectual intuition (inspectio mentis, eye of the soul). This intellectual intuition, along with deduction, constitutes the two operations of the mind (presented in Discourse on the Method). Intuition achieves evidence, establishing the criterion of certainty: what we consider obvious is always true.
Res Cogitans and Res Extensa
From the cogito, we conclude that we are a thinking thing (res cogitans). For Descartes, res cogitans is the soul, not in the Platonic, Aristotelian, or scholastic sense, but understood as a finite and thinking substance. Cartesian metaphysics derives the concept of substance (infinite and thinking) from God and then applies it analogously to created things (finite substances). Modifying St. Anselm’s ontological argument, God is a perfect, infinite substance whose essence implies existence. Created finite substances need God to exist and be preserved. They are conceived clearly and distinctly: the thinking substance (cogitans) in relation to the extended substance (extensa) and vice versa. God, the infinite thinking substance, is self-subsistent. Finite substances, although independently existing and conceivable from each other as cogitans and extensa, both need God to be and remain in being (even an accident like a memory or feeling needs res cogitans, because how could one think of something without an individual who thinks?). This analogy suggests that finite substances are complete in themselves but incomplete in isolation, forming the human composite. To explain the interaction of substances, Descartes uses the pineal gland (explaining, for example, the body’s voluntary movements through the soul’s passions). However, his anthropology rejects a strict dualism. For Descartes, body and soul are not like a pilot in a ship but are so intertwined that they are often confused (when the body suffers, we feel it not only intellectually but also physically). Descartes’s metaphysics studies substances, attributes (essence of the substance), and modes (qualifications of the substances). In short, there are three substances: God (infinite and thinking substance, perfect being, and self-subsistent), res cogitans (soul, finite and thinking substance that feels, knows, wants, ignores, and depends on God), and res extensa (material and extended finite substance characterized by figure, motion, rest, color, smell, taste, touch, etc., and that depends on God).