Descartes vs Hume — How Do They Compare?

Quick Answer

René Descartes (1596–1650) and David Hume (1711–1776) are the two poles of early modern epistemology. Descartes is the founder of modern rationalism, who sought to rebuild knowledge on a foundation of certainty by methodically doubting everything that could possibly be doubted, until he arrived at the one indubitable proposition: cogito ergo sum — "I think, therefore I am." Hume is the founder of modern empiricism in its most rigorous form, who argued that all human knowledge derives from sensory impressions and that the supposedly rational certainties Descartes prized — the self, causation, the external world — are in fact habits of association forged by repeated experience rather than deliverances of reason.

The two thinkers disagree about the source of knowledge (reason or experience), the existence of innate ideas (Descartes affirms them, Hume denies them), the status of causal inference (Descartes takes it to be rationally grounded, Hume takes it to be a matter of custom), and the nature of the self (Descartes finds a thinking substance, Hume finds a bundle of perceptions). Their disagreement set the agenda for Kant, who would later try to synthesize rationalist and empiricist insights in the Critique of Pure Reason, and it remains central to contemporary epistemology and philosophy of mind.

Core Distinction

The single sharpest difference is this: Descartes holds that genuine knowledge must be grounded in reason and attain certainty; Hume holds that all knowledge derives from experience and that certainty, in the domains where Descartes sought it, is unavailable.

For Descartes, the senses are unreliable (the stick that looks bent in water, the tower that looks round at a distance but square up close), and reasoning that depends on them is correspondingly insecure. To find anything indubitable, one must set aside everything that admits of any doubt — including the testimony of the senses, the deliverances of memory, and even the existence of one's own body — and see what remains. What remains is the thinking self: I cannot doubt that I am doubting, and so long as I am doubting, I exist. From this fixed point, Descartes attempts to reconstruct knowledge of God, the external world, and the principles of natural philosophy.

For Hume, the rationalist program is a failure because its fundamental premise is false. There are no innate ideas; the mind at birth is a tabula rasa (a thesis Hume inherits from Locke), and all the materials of thinking derive from impressions — the vivid perceptions of sensation and reflection. Ideas are faint copies of impressions. To determine whether a philosophical term has meaning, Hume proposes what he calls the "copy principle": "when we entertain… any suspicion that a philosophical term is employed without any meaning or idea… we need but enquire, from what impression is that supposed idea derived?" (Enquiry, II). Terms for which no impression can be found are suspect. Applied rigorously, this principle dissolves the rationalist's supposedly innate ideas — including the idea of substantial self and the idea of necessary causal connection — into nothing.

Descartes: Methodical Doubt and the Cogito

Descartes's epistemological project is laid out principally in the Meditations on First Philosophy (1641) and the Discourse on the Method (1637). The project begins with a decision to demolish one's opinions and rebuild from scratch, accepting only what survives the most radical doubt.

The method of doubt proceeds in stages. First, the senses deceive us about small and distant things; perhaps they deceive us about everything. Second, there is no decisive way to distinguish dreaming from waking — I may be dreaming that I am sitting by the fire when I am in fact asleep in bed. Third, and most radically, an omnipotent deceiver (the evil demon of Meditation I) could ensure that even my most certain mathematical beliefs (that 2 + 2 = 4) are false. Having pushed doubt to this extreme, Descartes finds one thing he cannot doubt: that he is thinking. Even if he is deceived, he must exist to be deceived; even if the deceiver is deceiving him about his own existence, he must exist to be deceived. The proposition "I am, I exist" is necessarily true every time I think it or pronounce it (Meditations II).

This is the cogito. From it Descartes draws a conclusion about what he is: a thinking thing (res cogitans) — a thing that doubts, affirms, denies, wills, refuses, imagines, and senses (in the sense of being aware of seeming-sensations). He is not, at this stage of the argument, certain that he has a body; that comes later, after the proof of God's existence.

The proof of God is the linchpin of the Cartesian system. Descartes argues that the idea of an infinite and perfect being cannot have originated in a finite and imperfect mind; the cause of an idea must contain at least as much reality as the idea itself, so only an actually infinite and perfect being could be the cause of the idea of such a being (Meditations III). A second, more famous argument turns on the necessity of God's existence: I have an idea of a perfect being, and existence is a perfection, so the perfect being must exist (the ontological argument of Meditations V). Because God is perfect, God is not a deceiver; therefore, what I clearly and distinctly perceive is true. This licenses the recovery of mathematical knowledge, knowledge of the external world (which God would not allow me to be systematically deceived about, provided I use my faculties carefully), and the principles of Cartesian natural philosophy.

The role of the cogito and the proof of God together constitute what is sometimes called the "Cartesian Circle": Descartes seems to need God's veracity to guarantee clear and distinct perception, but to prove God's existence he seems to rely on clear and distinct perception. The objection was raised by Arnauld in the Fourth Set of Objections and has haunted the Meditations ever since.

On causation, Descartes is a rationalist. God is the primary cause of all created reality, sustaining it in existence moment by moment. Created things have real causal powers — fire really does heat, the mind really does move the body (through the pineal gland, in Descartes's speculative physiology). Causal connections are intelligible to reason and are grounded in the natures of the things involved and ultimately in God. There is no question of causation being a mere habit of association; the world is a rationally structured system created by a non-deceiving God, and our clear and distinct ideas of it can attain certainty.

Hume: Impressions, Ideas, and Custom

Hume's empiricism is developed principally in A Treatise of Human Nature (1739–40) and the Enquiry Concerning Human Understanding (1748). Hume begins by dividing the contents of the mind into impressions — the vivid, forceful perceptions of sensation (red, hot, loud) and reflection (desire, aversion, joy) — and ideas, which are the fainter copies or memories of impressions. The copy principle demands that every meaningful idea be traceable to some impression.

The copy principle is not merely a psychological thesis about the origin of ideas; it is a criterion of meaning. Hume uses it as a sieve, sifting legitimate philosophical concepts from empty ones. When we encounter a term that purports to name something real — substance, necessary connection, the self — we ask what impression it derives from. If no impression can be found, the term is suspect.

Applied to causation, the principle is devastating. We commonly suppose that there is a necessary connection between cause and effect — that the fire makes the water boil, that the billiard ball forces the other to move. Hume asks what impression gives rise to the idea of this necessary connection. When we observe one event followed by another, what do we actually perceive? We perceive the first event, then the second event; we perceive their constant conjunction across repeated instances; we perceive no "power" or "necessity" linking them. The idea of necessary connection, Hume argues, is not derived from any impression in the objects; it is derived from an internal impression — the feeling of the mind's customary transition from one idea to another after repeated experience (Enquiry, VII). We feel determined to expect the second event after the first, and we project that feeling onto the objects, mistaking a habit of the mind for a feature of the world.

This is Hume's "skeptical solution" to the problem of induction. Reason alone cannot justify the inference from "I have seen the sun rise a thousand times" to "the sun will rise tomorrow" — there is no contradiction in supposing that the future will resemble the past, but neither is there any contradiction in supposing it will not. The inference rests not on reason but on custom or habit. "Custom," Hume writes, "is the great guide of human life" (Enquiry, V). This is not a counsel of despair but a recognition that the structure of human cognition is such that we cannot help forming expectations from repeated experience, and that this is what makes life and inquiry possible.

Applied to the self, the principle is equally devastating to the Cartesian inheritance. Descartes found the self as a thinking substance in the cogito. Hume asks what impression gives rise to the idea of a substantial self. He reports: "For my part, when I enter most intimately into what I call myself, I always stumble on some particular perception or other… I never can catch myself at any time without a perception, and never can observe anything but the perception" (Treatise I.iv.6). The self, for Hume, is not a substance but a bundle — a collection of perceptions succeeding one another with inconceivable rapidity, connected by relations of resemblance, contiguity, and causation (here, mental causation). The unity of the self is not a metaphysical fact but a psychological construction, the product of the mind's tendency to associate related perceptions and mistake the smooth transition among them for the perception of an underlying substance.

Hume's moral and practical philosophy follows the same empiricist method. Reason is "the slave of the passions" (Treatise II.iii.3): it can tell us how to get what we want, but it cannot tell us what to want. Moral distinctions are not derived from reason but from sentiment — our feelings of approval and disapproval, which are themselves natural responses shaped by sympathy, education, and the structure of human social life. This is a sharp departure from Descartes, for whom the will (a faculty of the soul) can assent or withhold assent to clear and distinct ideas and is central to moral evaluation.

Where They Agree

The disagreement is sharp, but the two thinkers share important commitments.

Both are skeptical of scholastic Aristotelianism. Descartes rejects substantial forms, real qualities, and the apparatus of medieval metaphysics as obscure and useless. Hume rejects them even more decisively, treating them as verbal confusions produced by failing to trace ideas to impressions. Both are part of the early modern revolt against the scholastic inheritance.

Both insist that philosophy must begin from a critical examination of the mind's capacities. Descartes's methodical doubt and Hume's science of human nature are both attempts to determine what the mind can and cannot know before building substantive theories. The Meditations and the Treatise are both, in their very different ways, investigations of the cognitive subject as a precondition for any further philosophy.

Both treat the new science of nature as a backdrop. Descartes is himself a major contributor to the new mechanical philosophy; Hume is its philosophical interpreter, seeking to understand what justification our scientific inferences actually have. Neither is doing natural philosophy in the medieval mode.

Both recognize the problem of the external world as a serious one. Descartes raises it in Meditation I and resolves it (he thinks) through the proof of God's veracity. Hume raises it in the section "Of skepticism with regard to the senses" (Treatise I.iv.2) and resolves it, if at all, by appeal to natural belief — a non-rational tendency of the mind to continue believing in the external world even after philosophical argument has undermined its rational basis.

Both are, in different ways, mitigated skeptics. Descartes uses radical doubt to arrive at a foundation that then permits the recovery of ordinary belief. Hume argues that radical skepticism is psychologically unlivable and that nature reasserts itself when philosophical doubt becomes unbearable — "carelessness and inattention alone can afford a remedy" (Treatise I.iv.2). Both end up, in practice, endorsing a form of moderate skepticism that does not paralyze inquiry.

Comparison Table

| Dimension | Descartes | Hume | |---|---|---| | Source of ideas | Innate ideas plus ideas derived from the senses; reason can generate ideas not derived from experience | All ideas derive from impressions (sensory or reflective); no innate ideas | | Method | Methodical doubt — reject whatever can be doubted; retain only the indubitable | Empirical investigation of mental contents; the copy principle as a criterion of meaning | | Foundation of knowledge | The cogito ("I think, therefore I am"), secured by the existence of a non-deceiving God | No indubitable foundation; knowledge rests on custom, habit, and natural belief | | Status of the self | A thinking substance (res cogitans) known with certainty in the cogito | A bundle of perceptions; no impression of a continuous substantial self | | Causation | Real causal powers grounded in the natures of things and ultimately in God; rationally intelligible | No impression of necessary connection in objects; causation is constant conjunction plus a felt habit of the mind | | Role of reason | Reason can attain certainty about God, the self, and the principles of nature | Reason is the slave of the passions; it can discover relations of ideas and matters of fact, but cannot motivate or ground morality | | External world | Recovered through God's veracity; the senses (properly used) reveal a real extended world | Belief in the external world is a natural belief, not rationally demonstrable | | Religious commitment | Defends the existence of God and the immortality of the soul | Skeptical of natural theology; critiques miracles and the design argument |

Why It Matters

The Descartes–Hume comparison matters because it defines the two great programs of modern epistemology and sets the problem that Kant's critical philosophy was created to solve.

Kant famously wrote that "it was David Hume who first interrupted my dogmatic slumber" (Prolegomena, Preface). Hume's attack on causation showed Kant that the rationalist project of deriving substantive knowledge from reason alone could not succeed; Descartes's attempt to ground knowledge in the cogito showed that empiricism alone could not account for the necessity and universality that mathematics and natural science seemed to possess. Kant's solution was to argue that the mind contributes the forms of intuition (space and time) and the categories (substance, causation, etc.) that structure experience, so that knowledge is possible only of appearances (phenomena) and not of things-in-themselves (noumena). The Critique of Pure Reason is, in effect, an attempt to reconcile Descartes's insistence on necessity with Hume's insistence that necessity cannot be derived from experience.

The comparison also matters for contemporary epistemology. The internalism/externalism debate about justification, the foundationalism/coherentism debate about the structure of knowledge, and the ongoing arguments about the problem of induction all inherit their terms from the Descartes–Hume dispute. Foundationalists who seek an indubitable basis for knowledge (Descartes) face Humean objections about whether such a basis can be found or, if found, can support the superstructure. Coherentists and naturalized epistemologists (in the tradition of Quine, who saw himself as extending Hume) argue that knowledge has no such foundation and that epistemology should be continuous with empirical psychology.

In philosophy of mind, the bundle theory of the self that Hume proposes is a direct ancestor of contemporary "no-self" or "illusionist" views about personal identity, while the Cartesian view that the self is a thinking substance survives in dualist theories of mind. The contemporary debate between physicalists and dualists, and within physicalism between those who allow a robust first-person perspective and those who would dissolve it, is in part a continuation of the Descartes–Hume dispute about the self.

Finally, the comparison matters for understanding the limits of reason itself. Descartes's confidence that reason can attain certainty and Hume's insistence that reason's reach is far more limited than its admirers suppose are not merely historical positions; they are live options that every reflective person must confront. The question of what we can know, and how, is not one that philosophy has solved and set aside.

Common Questions

Did Hume refute Descartes? In one sense, yes: Hume's arguments against innate ideas, against the rational intelligibility of causation, and against the substantial self are widely regarded as decisive against the specifics of the Cartesian system. In another sense, no: Hume's empiricism cannot account for the necessity of mathematics and logic (he treats them as relations of ideas, but the question of what grounds their necessity remains open), and his own bundle theory of the self faces well-known objections (how can a bundle of distinct perceptions be aware of itself as a unity?). Kant's critical philosophy is in large part an attempt to preserve what is right in both Descartes and Hume while avoiding what is wrong in each.

What is the "problem of induction" and how does it relate to Descartes? The problem of induction, as Hume formulates it, is that there is no non-circular rational justification for inferring that the future will resemble the past or that unobserved cases will resemble observed ones. Descartes thought he could escape such problems by grounding knowledge in God's veracity, so that our faculties, properly used, would not systematically mislead us. Hume rejects this escape route: the existence of God cannot be established by reason alone (the cosmological and ontological arguments fail), and even if it could, it would not justify inductive inference without circularity. The problem of induction is, in part, the price of giving up the Cartesian foundation.

Were they both skeptics? Both were skeptics in a qualified sense, but the qualifications differ. Descartes is a methodological skeptic: he employs doubt as a technique for arriving at certainty, and his skepticism is provisional and overcome. Hume is a mitigated or "academic" skeptic: he holds that radical doubt is psychologically unsustainable (nature reasserts itself) but that philosophy should curb the pretensions of reason, reminding us how little of what we believe can be rationally justified. Descartes's skepticism is a ladder to be climbed and then discarded; Hume's is a permanent posture of intellectual modesty.

Sources

• Descartes, René. Meditations on First Philosophy (1641), translated by John Cottingham (Cambridge University Press, 1996).
• Descartes, René. Discourse on the Method (1637), translated by John Cottingham et al. (Cambridge University Press, 1985).
• Hume, David. A Treatise of Human Nature (1739–40), edited by David Fate Norton and Mary J. Norton (Oxford University Press, 2000).
• Hume, David. An Enquiry Concerning Human Understanding (1748), edited by Peter Millican (Oxford University Press, 2007).
• René Descartes — Stanford Encyclopedia of Philosophy: https://plato.stanford.edu/entries/descartes/
• David Hume — Stanford Encyclopedia of Philosophy: https://plato.stanford.edu/entries/hume/
• Descartes — Internet Encyclopedia of Philosophy: https://iep.utm.edu/descarte/
• Bennett, Jonathan. Learning from Six Philosophers: Descartes, Spinoza, Leibniz, Locke, Berkeley, Hume (Oxford University Press, 2001).