When I first came across Descartes and Hume in my reading, it struck me how both of them start from a kind of uncertainty that feels familiar from everyday life. You work something out in your head and it seems solid, yet what you actually see or feel pulls in another direction. That gap between thinking and sensing runs right through their work. This essay looks at where they differ on the origins of knowledge, how they treat reason against sense perception, and how far each thinks certainty can stretch.
Descartes and the Limits of the Senses
Descartes sets out by treating almost everything the senses offer as doubtful. In the Meditations he asks what would happen if he supposed an evil demon was feeding him false experiences. The thought experiment is extreme, yet it grows out of ordinary moments when perception lets us down. Phantom limb pain is one everyday reminder: someone who has lost a leg can still feel it as clearly as before. Nothing in the current sensory field explains the sensation, so reliance on the body’s reports begins to look shaky.
From that starting-point Descartes moves toward what he calls clear and distinct perceptions. Once he reaches the certainty of his own thinking—“I am thinking, therefore I exist”—he builds outward again. Reason, not the senses, becomes the route to trustworthy knowledge. The famous wax example illustrates the point. A piece of wax looks, smells and feels one way when cold, yet after it is warmed everything changes. The senses register a completely different object, yet we still know it is the same wax. Descartes argues that this knowledge comes from the intellect judging the underlying substance, not from the shifting impressions of sight or touch. In this way innate ideas, such as the concept of extended matter, seem to be part of the mind from the start and do not depend on experience.
He therefore allows some certainty, provided it rests on reason’s clear and distinct grasp rather than on fallible sensation. Dreams fit the same pattern: while dreaming we often accept the scene as real, and only later reflection reveals the mistake. Descartes treats this as further evidence that the senses alone cannot guarantee truth.
Hume’s Turn to Experience and Custom
Hume begins from the opposite direction. All our mental content, he claims, traces back to impressions received through the senses or inner feeling. Ideas are simply fainter copies of those impressions. Without an impression, no genuine idea can form. This rules out the sort of innate ideas Descartes relies on. Concepts such as substance or necessary connection are not discovered by pure reason; they arise after repeated experiences have built up expectations.
Causation shows the difference sharply. We see one billiard ball strike another and the second move, and we come to expect the connection every time. Hume insists this expectation rests on habit or custom, not on any insight reason could supply. We cannot observe a necessary link between the two events; we only notice constant conjunction. Because knowledge stays tied to what experience has so far delivered, we can never be certain that the pattern will continue. The sun has risen every morning we remember, yet that gives no logical proof it must rise tomorrow.
Sense perception therefore supplies the raw material, while reason is limited to rearranging and comparing ideas already derived from experience. Certainty, in Hume’s view, is mostly restricted to relations of ideas (as in mathematics) or to the immediate contents of current impressions. Everything else remains probable at best.
Contrasting Reason, Perception and Certainty
The two thinkers therefore part company on how much trust to place in reason versus the senses. Descartes uses radical doubt to clear away sensory evidence, then rebuilds knowledge through innate ideas and clear reasoning; what survives is what intellect can grasp distinctly. Hume treats experience as the sole source and sees reason as working within the limits set by past impressions; any claim beyond those limits lacks secure foundation. Where Descartes finds certainty in the intellect’s grasp of substance despite changing appearances, Hume finds only a useful habit formed by repeated impressions.
These differences also shape what each philosopher allows us to know for sure. Descartes keeps room for certainty about the self, God and material extension once reason has done its work. Hume leaves us with far less: we can know the relations between our current ideas and the regularities experience has taught us, but we cannot claim necessity or reach beyond the stream of impressions. The tension between reason and experience therefore ends in opposite places—one anchored in what the mind brings to experience, the other in what experience alone can furnish.
Conclusion
Descartes and Hume both recognise that perception can mislead, yet they draw opposite conclusions about the source of reliable knowledge. Descartes rescues certainty by turning to innate ideas and rational insight; Hume ties every idea to prior impressions and accepts that custom rather than necessity governs most of our beliefs. Their disagreement continues to mark the boundary between rationalist and empiricist approaches, and it still shapes how we weigh what we think against what we actually encounter.
References
- Descartes, R. (1641) Meditations on First Philosophy. Translated by J. Cottingham (1996). Cambridge: Cambridge University Press.
- Hume, D. (1748) An Enquiry Concerning Human Understanding. Edited by T. L. Beauchamp (1999). Oxford: Oxford University Press.
- Cottingham, J. (1986) Descartes. Oxford: Blackwell.
- Stroud, B. (1977) Hume. London: Routledge.
