René Descartes's project in the Meditations on First Philosophy (1641) was audacious: he would accept as true nothing that he could doubt, however slightly, and see what survived. He doubted the senses (they deceive us). He doubted the external world (he might be dreaming). He doubted mathematics (an evil demon might be causing him to make mistakes in calculation). After this radical demolition, one thing remained: the act of doubting itself. "Cogito ergo sum" — I think, therefore I am. The doubter cannot doubt that he is doubting.
From this solitary certainty, Descartes attempted to reconstruct the rest of knowledge by pure deductive reasoning. He proved (to his own satisfaction) the existence of God, then used God's non-deceptiveness to guarantee the external world and the reliability of clear and distinct ideas. The entire project was rationalist: reason, not experience, was the source of genuine knowledge. This framework established the problem of epistemology as the central concern of modern philosophy — the question of how we know anything at all.
David Hume's empiricism took Locke and Berkeley's insight — that all knowledge derives from sensory experience — to its most radical conclusion. In the Treatise of Human Nature (1738) and the Enquiry Concerning Human Understanding (1748), Hume argued that human reason, far from being an independent faculty that grasps eternal truths, is merely the association of ideas that have been connected by experience.
His most devastating move was against causality — the cornerstone of both common sense and Newtonian science. When we say that A causes B, Hume asked, what do we actually observe? We observe A, then B, repeatedly. We do not observe any necessary connection between them — any "must" or "always will." The expectation that the future will resemble the past is not a logical necessity; it is a psychological habit formed by repetition. We expect the sun to rise because it always has, not because we can logically prove it always will. This "problem of induction" remains one of philosophy's genuinely unresolved problems.
Descartes's entire structure rested on the cogito — the self that thinks. Hume targeted this directly. When he introspected, Hume reported, he never caught himself at any time without a perception: some sensation, some emotion, some thought. He never found a bare "self" separate from the stream of experience — only the stream itself. The self is not a substance or a foundation; it is "a bundle or collection of different perceptions, which succeed each other with inconceivable rapidity."
If the self is a bundle rather than a substance, Descartes's "I think, therefore I am" proves only that thinking is happening — not that there is a substantial "I" doing it. The foundation on which Descartes built his entire edifice dissolves into a succession of perceptions with no owner. This was more than a technical philosophical objection; it was an assault on the concept of personal identity, on moral responsibility, and on the rational agent that Enlightenment political philosophy assumed at its center.
Kant famously acknowledged that Hume "woke him from his dogmatic slumber." The problem Hume posed — if causality is just habit, how can Newtonian science claim objective validity? — became the animating question of Kant's Critique of Pure Reason (1781). Kant's answer was that causality and other fundamental principles are not derived from experience but are structures imposed by the mind on experience — they are "a priori synthetic" categories that organize the raw flux of sensation into a comprehensible world. This transcendental idealism was Kant's attempt to save the objectivity of science while acknowledging Hume's empiricist insights.
The debate between Descartes's rationalism and Hume's empiricism thus generated Kant's synthesis, which in turn generated German Idealism, which in turn generated Hegel, Marx, phenomenology, analytic philosophy's reaction against all of it — an intellectual chain reaction that defines Western philosophy's last three centuries.
| Category | René Descartes | David Hume |
|---|---|---|
| Born | 1596, La Haye, France | 1711, Edinburgh, Scotland |
| Field | Philosophy, mathematics, physics | Philosophy, history, economics |
| IQ (est.) | ~185 | ~180 |
| Greatest Work | Meditations on First Philosophy; coordinate geometry | Treatise of Human Nature; Enquiry Concerning Human Understanding |
| Legacy | Modern philosophy's founding problem; coordinate geometry; dualism | Problem of induction; bundle theory; naturalized epistemology |
| Influence | Spinoza, Leibniz, Kant (against); all modern philosophy | Kant, Adam Smith, logical positivism, modern epistemology |
Hume is the more honest philosopher; Descartes is the more consequential one. Hume's skepticism is more internally consistent and his conclusions about induction and causality remain genuinely unrefuted — no one has solved the problem of induction, and Hume's bundle theory of self is arguably closer to what neuroscience suggests than Descartes's substantial ego.
Yet Descartes's move — asking how we know anything and insisting on certainty as the standard — set the agenda for three centuries of philosophy. Without Descartes's question, there is no Hume's answer, no Kant's synthesis, no modern epistemology at all. Descartes built the house; Hume pointed out it had no foundation. Kant spent his life trying to fix the plumbing. The project continues.