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In the Meditations, Descartes employs a method of systematic doubt: he seeks to identify beliefs that cannot possibly be doubted, even under the most extreme hypothetical scenarios. He imagines a supremely powerful evil demon that might be deceiving him about all of his perceptual experiences and even about mathematical reasoning. This skeptical hypothesis is designed to identify which beliefs, if any, are immune to all possible doubt. Descartes believes he finds bedrock in the cogito — 'I think, therefore I am' — since the very act of doubting confirms that something is doing the doubting.
Read Meditations I–II closely, tracking which beliefs Descartes suspends and why. Then consider whether the evil demon hypothesis is a defeater for every belief or only some. Compare the evil demon to its modern cousin, the brain-in-a-vat scenario.
You already know from your study of knowledge that justified true belief — whatever its precise formulation — requires that the believer have some adequate grounds for the belief. Descartes's project in the Meditations begins from a different question: not "what is knowledge?" but "what can survive the most extreme possible assault on justification?" He is not claiming to doubt everything because he actually doubts it. The method of doubt is a tool, not a psychological state — a thought experiment designed to test beliefs by imagining the most hostile possible epistemic circumstances.
The escalation of the doubt proceeds in stages. First, Descartes notices that his senses have sometimes deceived him — objects appear different at a distance, illusions occur. This observation is enough to suspend trust in any particular sensory belief, but it does not undermine all sensory knowledge. He escalates: perhaps he is dreaming right now, in which case even the most vivid apparent perceptions could be false. This is broader but still limited: even in dreams, he seems to be having experiences with some structure. The decisive move is the evil demon hypothesis. Suppose a supremely powerful being has been systematically deceiving him about everything — not just perceptions, but even mathematical and logical truths that seem self-evidently necessary. Could 2 + 2 = 4 be a deception? Under this hypothesis, almost any belief could be false.
This is where your background in thought experiments is essential. The evil demon is not a live hypothesis Descartes believes to be true — it is a skeptical scenario, a worst-case construction whose function is to pressure-test beliefs by revealing which ones could not survive even this extreme attack. A belief that might be false under the evil demon scenario cannot count as certain. Descartes is searching for beliefs that are demon-proof: beliefs such that even if the demon exists, it cannot make them false. He finds one: *cogito ergo sum* — "I think, therefore I am." The very act of doubting requires that something is doing the doubting. The demon can deceive you about external objects, mathematics, your own past — but it cannot deceive you into thinking you are thinking when nothing is. Thinking is self-verifying in a way that perception is not.
The modern descendant of the evil demon is the brain-in-a-vat scenario: suppose your brain has been removed from your body, placed in a vat of nutrients, and connected to a supercomputer that feeds you a perfectly simulated reality. All your experiences are the same as they would be if the world were real — you have no way from the inside to detect the deception. This scenario has the same structure as the evil demon and raises the same question: if you cannot rule it out, do you know anything about the external world? Descartes's answer is to escape through the cogito and then rebuild from there. Whether that reconstruction succeeds is the problem for the topics ahead.
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