al-Ghazali’s doubt

Al-Ghazālī (1058–1111), born in Tus in Persia, became one of the most famous scholars of his age. Late in his life he wrote Deliverance from Error, the story of how he lost his certainty and got it back. He wanted knowledge so sure that nothing could shake it. So he tested the two things he trusted most, the senses and reason.

The senses failed first. You look at a shadow and it seems to stand still, but an hour later it has moved. You look at a star and it seems the size of a coin, but geometry shows it is bigger than the earth. If the senses can be corrected by reason, maybe reason can be corrected by something above it.

Then he lets the senses answer back. Don’t you see, they say, that in sleep you believe things and see whole situations, and you’re sure they’re solid and lasting? Then you wake up and know none of it had any basis. What makes you sure that waking life isn’t the same, compared with some state beyond it? From there your waking certainties would look the way dreams look from here. He notes that the Sufis claim to reach such states, and he quotes the saying “People are asleep, and when they die they wake up.”

He couldn’t argue his way out, because any proof would have to use the reason that was in question. For about two months, he says, he was a sceptic in fact, if not in speech. What ended it wasn’t an argument. It was, in his words, a light God cast into his breast, which gave him back his trust in the basic truths of reason.

That’s why the passage matters for the simulation hypothesis. It’s the cleanest early statement of the problem: from inside one level, you can’t prove there is no level above it. Al-Ghazālī accepts that, and gets out by grace, not by logic. Descartes, five centuries later, would try to get out by logic.