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René Descartes (1596–1650) is widely regarded as the father of modern philosophy. A French mathematician, scientist and philosopher, Descartes broke decisively with the medieval Aristotelian–Thomistic tradition and sought to rebuild human knowledge from the ground up on a foundation of absolute certainty. His method of radical doubt, his discovery of the cogito, his substance dualism and his arguments for God's existence are all central to AQA A-Level Religious Studies, especially the topics of the soul and of arguments for God. His principal philosophical works are the Meditations on First Philosophy (Meditationes de Prima Philosophia, 1641), the Discourse on Method (1637) and the Principles of Philosophy (1644).
In the Meditations Descartes set out to find a proposition that is absolutely certain — immune to any conceivable doubt. To do so he employed a method of systematic, hyperbolic doubt: he resolved to treat as false anything that could be doubted, however unlikely the grounds. His aim was not to embrace scepticism but to find an indubitable foundation on which to rebuild knowledge — like an architect who tears a house down to the bedrock before building securely.
Key term: methodological doubt. The deliberate, temporary suspension of belief in anything open to doubt, used not to end in scepticism but as a tool to discover whatever survives doubt and can serve as a secure foundation for knowledge.
Descartes identified three progressively more radical grounds for doubt:
The evil-demon hypothesis is the most extreme form of doubt. If even mathematics and logic can be doubted, what — if anything — survives?
It is worth being clear about the strategy here, because students sometimes mistake Descartes for a sceptic. His doubt is hyperbolic (exaggerated) and methodological (a means to an end): he doubts not because he genuinely believes the demon exists, but because a foundation that can survive even that extravagant supposition will be unshakeable. He also doubts wholesale rather than belief-by-belief — undermining whole classes of belief (everything from the senses, everything that could be dreamt) — which is far more efficient than examining each belief individually. The three grounds form a deliberate escalation: the senses-argument unseats particular perceptions; the dream-argument unseats the existence of the external world and the body; the demon-argument unseats even the eternal truths of mathematics and logic. Only against this maximally corrosive background does the survival of the cogito carry its full force — it is certain even if I am dreaming, even if a demon bends all my reasoning, because the very act of being deceived requires that I exist to be deceived.
In the Second Meditation Descartes found the one proposition that survives even the most radical doubt:
"I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." — Descartes, Meditations, II
This is the famous cogito (from the Latin cogito, ergo sum — "I think, therefore I am," a formulation that appears in the Discourse on Method). The reasoning is deceptively simple but profound:
The cogito is therefore self-verifying: any attempt to doubt it confirms it, since doubting is thinking and thinking presupposes a thinker. Descartes concluded that the cogito provides an Archimedean point — a single fixed, immovable foundation on which all other knowledge can be rebuilt. He also concluded that he is, in his essential nature, a thinking thing (res cogitans) — a mind or consciousness whose existence is more certain than anything in the physical world.
The cogito is not beyond challenge, and a good evaluation notes the objections. The physicist-philosopher Georg Lichtenberg famously argued that Descartes is entitled only to "there is thinking going on," not to "I think": the move to a single, persisting self who does the thinking smuggles in more than the bare experience licenses, since all that is strictly given is the occurrence of thought. Bertrand Russell pressed the same point — the certainty extends to the present thought, not to a substantial thinker enduring through time. If these objections hold, the cogito secures something thinner than Descartes supposed: the reality of thinking, but not yet an immaterial substance of the kind his dualism needs. Descartes's reply would be that thought cannot be a free-floating property with no subject — but whether that is a discovery or merely a habit of grammar is precisely what is in dispute.
A subtle but important point: the cogito is not, for Descartes, an inference in the ordinary sense (a syllogism with a hidden premise "whatever thinks exists"). It is a direct, intuitive certainty grasped in the very act of thinking. This is why it can stand even when the reliability of reasoning is itself in doubt — it is seen, not deduced.
Immediately after the cogito, in the Second Meditation, Descartes presses a further point with the famous wax argument, which AQA candidates can use to show how his rationalism works. Consider a piece of beeswax fresh from the comb: it has a certain colour, scent, shape, hardness and sound when tapped. Bring it near a fire and every one of these sensory qualities changes — it melts, loses its scent, alters its shape and colour. Yet we judge it to be the same wax. How? Not through the senses, since all the sensible qualities have changed, and not through imagination, since I cannot imagine the indefinitely many shapes the wax could take. I grasp that it is the same wax through the intellect alone — by understanding it as an extended, flexible, changeable substance underlying the shifting appearances.
The lesson is twofold and deeply rationalist. First, even bodies are known more truly by the mind than by the senses: what is stable and knowable in the wax is the idea of extension, grasped intellectually rather than perceived. Second — and this is the sting Descartes intends — every time I perceive the wax, I am even more certain of my own existence as the perceiver than of the wax, since the perception itself confirms the cogito. The mind, he concludes, is "better known than the body." This is the exact reverse of the empiricist order of priority that Hume would later defend, and the contrast between the two is one of the most fertile in the whole of philosophy of religion.
Key term: clear and distinct ideas. Descartes's criterion of truth: an idea is clear if it is present and accessible to an attentive mind, and distinct if it is sharply separated from all other ideas. Descartes holds that whatever he perceives clearly and distinctly is true — a rule the cogito exemplifies and which God's non-deceiving nature is ultimately invoked to guarantee.
From the cogito Descartes developed his theory of substance dualism — the view that reality contains two fundamentally different kinds of substance:
Key term: substance dualism. The view that mind and body are two distinct substances of fundamentally different natures (thinking versus extended), each capable in principle of existing without the other — as opposed to materialism (only matter exists) or property dualism (one substance, two kinds of property).
Descartes argued that mind and body are really distinct — each can exist without the other — via what is often called the conceivability argument:
He gives a supporting argument from divisibility: the body, being extended, is divisible into parts, whereas the mind is indivisible — I cannot conceive of half a mind, or of my consciousness being literally cut in two. Since they have contradictory properties (divisible / indivisible), and one thing cannot have contradictory properties, mind and body must be distinct. Critics question the premise: cases of split-brain patients (where severing the connection between the hemispheres can produce strikingly divided behaviour) and of dissociative conditions have been used to argue that consciousness may not be as seamlessly indivisible as Descartes assumed, which would weaken this argument. A Cartesian can reply that even apparently "split" consciousness is still experienced from a single point of view at any moment — but the dispute shows that even Descartes's supporting arguments rest on contestable claims about the nature of mind rather than on self-evident truths.
Despite their distinctness, Descartes insisted that mind and body are intimately united in a human being. In the Sixth Meditation he wrote that the mind is not lodged in the body merely "as a sailor in a ship" but is "very closely joined and, as it were, intermingled with it," so that mind and body together form a genuine unit — which is why I feel pain as mine rather than merely observing damage to my body.
Descartes's dualism faces a formidable objection, the interaction problem, pressed most acutely by Princess Elisabeth of Bohemia in her correspondence with him (1643):
"How can the soul of a man determine the spirits of his body so as to produce voluntary actions, given that the soul is only a thinking substance?" — Princess Elisabeth to Descartes, 6 May 1643
The interaction problem remains one of the most serious objections to substance dualism and motivated alternative theories: materialism (only physical stuff exists), idealism (only mental stuff exists), occasionalism (God coordinates mind and body on each occasion), parallelism (the two run in pre-established harmony without interacting), and property dualism (one substance with both physical and mental properties).
It would be a mistake to think dualism is merely a historical curiosity refuted by the interaction problem; AQA rewards candidates who can state its enduring strengths as well as its weaknesses.
In its favour: dualism takes seriously the first-person character of consciousness — the fact that there is "something it is like" to feel pain or see red, which no description of neurons obviously captures (the modern "hard problem of consciousness"). It coheres with the widespread intuition that I am not identical to my body, that my thoughts are private in a way physical objects are not, and with religious hopes of surviving bodily death. It also fits the apparent unity and indivisibility of the conscious self.
Against it: besides the interaction problem, dualism struggles with the overwhelming evidence from neuroscience that mental states depend intimately on brain states — damage the brain and the mind is altered, which looks more like dependence than mere interaction. It faces the problem of other minds (if minds are private non-physical substances, how do I know anyone else has one?) and the difficulty of explaining how an immaterial soul could be individuated — what makes my soul mine rather than yours, if souls have no spatial location? Materialists argue that the simplest hypothesis, given the brain's evident role, is that the mind just is the brain in action; dualists reply that no amount of physical detail explains why it is accompanied by experience at all. The debate is unresolved, which is exactly why Descartes's challenge still sets the terms.
In the Third Meditation Descartes presented the Trademark Argument (or Causal Argument) for God's existence. Having established that he exists as a thinking thing, Descartes asked what else he could know. He turned to his ideas, arguing that the idea of God must have a cause adequate to its content:
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