Descartes tried to doubt everything he possibly could, to find out what was left standing. Almost nothing was, and the one thing that survived changed philosophy.
1The puzzle
In 1641 René Descartes decided to doubt everything it was possible to doubt, to find what was left. Here is a tower of things you probably believe. Try to doubt each one. If you can find any reason at all, however unlikely, knock it down.
The sun will rise tomorrow.Only because it always has. Nothing proves the future must resemble the past.
This room exists around me.Dream rooms feel just as real while you’re in them.
I have two hands.You have hands in dreams too. Can you prove you’re not dreaming now?
My memories of yesterday are real.False memories feel exactly like true ones from the inside.
Other people have minds like mine.You only ever see their behaviour. A perfect imitation would look the same.
2 + 3 = 5.A perfect deceiver could make you feel certain of a wrong sum, and you’d never notice.
I am thinking right now.
7 beliefs standing.
2Take a stand
Is there anything you can be absolutely, 100% certain of?
There’s no right answer here. Pick the one closest to what you actually think. We’ll test it next.
3The push-back
A few situations designed to test your answer. After each one, you can hold your ground or shift. Watch where you end up.
4The map of answers
You’re not the first to wrestle with this. Tap a thinker to see their answer and the strongest case for it. Open at least two.
5Turn it on yourself
Most of what you know is not certain, and that’s fine. The danger is in the beliefs you treat as certain when they’re only familiar.
Your answers stay in this browser. Nobody else sees them.
6Your question card
Descartes’ doubt
What can you be absolutely certain of?
What is left when you doubt everything you can, and how much certainty do you actually need to act?
The main answers
G. E. Moore: I know I have hands. That is more certain than any clever argument against it.
Ludwig Wittgenstein: Some beliefs are the hinges that doubt turns on. They can’t meaningfully be doubted.
Charles Sanders Peirce: Doubt only matters when there is a real reason for it. Fake doubt is a game.
René Descartes: Doubt everything you can. What survives is certain: I think, therefore I am.
David Hume: Reason can’t justify most of what we believe, such as that the future will resemble the past. We believe it out of habit.
Pyrrho of Elis: For every argument there is an equal argument against. Suspend judgement.
Where you stand
Your note
Notice this week
Notice the next time you say “I’m certain”. Ask whether you mean certain, or just very confident.