René Descartes
René Descartes (1596–1650) was a French philosopher and mathematician, widely called the "father of modern philosophy" and a key figure of the Scientific Revolution. Refusing to accept the authority of the philosophers before him, he tried to rebuild knowledge from the ground up on foundations that could not be doubted.
Core ideas
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Methodic doubt & foundationalism. In the Meditations on First Philosophy and the Discourse on the Method, Descartes doubts everything that can possibly be doubted, seeking an indubitable starting point — accepting nothing as true unless it is "so clearly and distinctly" grasped as to exclude all doubt.
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The cogito. That starting point is usually given as cogito, ergo sum — "I think, therefore I am": even a universal deceiver cannot make me doubt that I, the doubter, exist. The thinking self becomes the first certainty and the ground of modern epistemology. Corrected 2026-08-21 from the primary text. That formula is from the Discourse (1637) and the Principles (1644). [[meditations-on-first-philosophy|The Meditations itself]] contains no such inference: what it establishes is that "this proposition (pronunciatum) I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind" — a certainty tied to each occasion of thinking it, not a syllogism. Descartes adds the restriction himself: "how often? As often as I think." Both readings are kept; the summary is not wrong about Descartes, it is wrong about which book.
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Mind–body dualism. Descartes divides reality into two substances — res cogitans (thinking mind) and res extensa (extended body/matter). The self is essentially a thinking thing, distinct from the body; how the two interact is the problem he bequeaths to everyone after. In the Meditations the definition of that thinking thing is a list, and a wider one than "intellect": "a thing that doubts, understands, [conceives], affirms, denies, wills, refuses; that imagines also, and perceives."
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Rationalism. Certain knowledge comes from reason, not the senses — the intellect, properly disciplined, is the source of truth.
Beyond philosophy
Descartes is also a founder of analytic geometry: the Cartesian coordinate system is named for him, bridging algebra and geometry (a mathematics contribution, distinct from his philosophy — cross-domain, not this spoke's metaphysics).
In the conversation
He is the origin point the others answer. Spinoza accepts his rationalist method but rejects his dualism for a one-substance metaphysics; Heidegger rejects the whole picture of a detached subject representing an external world (see mind-body-problem). Source: Wikipedia (T2, tertiary). The Meditations is now paged directly — see meditations-on-first-philosophy — so claims about that work should be taken from there, and this page kept for the life and the position in the conversation.
Related
meditations-on-first-philosophy · rationalism · mind-body-problem · baruch-spinoza · martin-heidegger · western-philosophy · michel-foucault