Rationalism and Descartes: the method of doubt, the three waves of doubt (the senses, the dream argument, the evil demon), the cogito as the first certainty, and the rationalist claim that reason is the foundation of knowledge.

What this dot point is asking

Descartes is the rationalist set thinker for Knowledge and Doubt. You must explain his method of doubt, the three waves of doubt he uses to test his beliefs, the cogito ("I think, therefore I am") that survives the doubt, and how this supports the rationalist claim that reason, not the senses, is the foundation of knowledge. This is the positive rationalist answer to the sceptical challenge.

The method of doubt

The aim is not scepticism for its own sake but certainty: Descartes wants a secure foundation on which to rebuild knowledge, like clearing away rubble before laying solid foundations. The standard is very high, indubitability, so any belief with even a possible source of error is set aside.

The three waves of doubt

The doubt escalates through three stages, each casting doubt on more than the last.

• Wave 1: the argument from illusion. The senses sometimes deceive (mirages, the bent stick in water), so beliefs based on the senses cannot be wholly trusted.
• Wave 2: the dream argument. At any moment Descartes cannot be certain he is not dreaming, since dreams can seem just as vivid as waking life. So even apparently obvious sensory beliefs ("I am sitting by the fire") could be false.
• Wave 3: the evil demon. Suppose an all-powerful malicious demon devotes itself to deceiving him about everything, including even simple truths like 2 + 3 = 5. Then nothing, neither the senses nor even reason, seems safe.

The cogito

The cogito is indubitable because doubting it confirms it. If the demon deceives Descartes, then Descartes is being deceived, and being deceived is a kind of thinking; thinking requires a thinker; so he must exist whenever he thinks. The more he tries to doubt his own existence, the more certain it becomes, because the doubting is itself thinking. The cogito is grasped by reason alone, clearly and distinctly, with no help from the senses, which is exactly why it serves Descartes' rationalism: it shows that the most certain knowledge comes from reason.

Rationalism

Rationalism is the view that reason is the primary source of knowledge, and that some knowledge is a priori (independent of experience). Descartes builds outward from the cogito using clear and distinct ideas, the marks of truth that reason can recognise. The contrast is with empiricism, which holds that knowledge comes ultimately from the senses, the position examined through Hume. Descartes' project is the rationalist reply to scepticism: by doubting everything and finding a rational certainty, he claims to rebuild knowledge on a foundation the sceptic cannot shake.

Examples in context

Consider how the cogito answers the sceptic from the first dot point. The sceptic said we cannot rule out the evil demon, so we cannot be certain of anything. Descartes agrees about the external world but turns the doubt on itself: "Let the demon deceive me as much as he can; he can never make me nothing while I think I am something." Here the very condition of being deceived, that there is an "I" doing the thinking, becomes the one thing certain. Notice the move: Descartes does not refute the demon directly; he finds a belief whose truth is guaranteed by the act of doubting it. That self-verifying structure is what makes the cogito the bedrock of his system.

Try this

Q1. What is Descartes trying to achieve with the method of doubt? [2 marks]

• Cue. To find a belief that cannot be doubted at all, an indubitable foundation, by rejecting as false anything open to the slightest doubt.

Q2. Why can the evil demon not make Descartes doubt the cogito? [2 marks]

• Cue. Being deceived by the demon is a kind of thinking, and thinking requires a thinker, so the deception itself proves that Descartes exists while he thinks.