The big idea: You think you know loads of things — that you're reading this, that the sun will rise, that 2 + 2 = 4.
The sceptic's question: But stop and ask: how do you actually know any of it? A sceptic presses that question until it hurts — and asks whether you can really be sure of anything at all.
This is scepticism. It isn't just doubting one fact; it's doubting whether real knowledge is possible in the first place. The point isn't to make you give up — it's to test how solid your knowledge really is.
Hold onto this: The sceptic isn't saying 'everything is false'. They're asking a sharper question: can you ever be certain enough to call something knowledge?
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One philosopher turned scepticism into a tool rather than a dead end.
Descartes' method of doubt: René Descartes tried a bold experiment: throw out every belief that could possibly be false, and see what's left standing. He wasn't trying to end up with nothing — he wanted to find a rock-solid foundation that no doubt could shake. He gives two famous reasons to doubt, each one bigger than the last.
1. The dream argument
Right now it feels obvious you're awake. But you've had dreams that felt completely real while you were in them. So how can you be sure you're not dreaming this very moment? If you can't rule it out, then everything your senses tell you might be a dream — and the whole outside world is in doubt.
Checkpoint — the dream: In one line: you can't prove you're awake right now, so your senses can't give you certainty. Descartes then makes the doubt even deeper.
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The dream still leaves maths untouched — 2 + 2 = 4 even in a dream. So Descartes invents a doubt big enough to reach even that.
2. The evil demon: Imagine an all-powerful evil demon whose whole aim is to trick you — feeding you a fake world and even making you get sums wrong while feeling sure you're right. If that were happening, you'd never notice.
So now *everything* is in doubt
- The world
- Your body
- Even simple maths
- It's the old version of 'what if you're in a simulation?'
The one thing that survives: 'I think, therefore I am': Then Descartes spots the escape. Even if the demon deceives you about everything, something has to be there to be deceived. To be tricked, you must be thinking — and if you're thinking, you exist. That's 'I think, therefore I am': the one belief no demon can fake away. It becomes his rock-solid foundation.
Go further — higher-level insight: Notice how narrow Descartes' certain foundation is. 'I exist as a thinking thing' is rock-solid — but getting from there back out to a real world, other people, or reliable senses is the hard part, and many think he never fully earns it. Naming that gap is a top-band move.
What he beats
- Scepticism about himself
What he leaves open
- The gap to the world, wide open
Checkpoint — bedrock: In one line: you can doubt everything except that you're thinking — because doubting IS thinking. That single certainty is meant to rebuild knowledge from the ground up.
Step back and question Descartes' hidden assumption: that unless you're certain, you don't really know.
Checkpoint — two replies: So there are two ways to answer the sceptic. (1) Meet the bar: find one certain thing and rebuild (Descartes). (2) Lower the bar: knowledge is strong justified belief, not 100% certainty. A good essay weighs both.