Paul Graham (Hackers & Painters) remembers little of what he’s read:
I’ve read Villehardouin’s chronicle of the Fourth Crusade at least two times, maybe three. And yet if I had to write down everything I remember from it, I doubt it would amount to much more than a page. Multiply this times several hundred, and I get an uneasy feeling when I look at my bookshelves. What use is it to read all these books if I remember so little from them?
[...]
Reading and experience train your model of the world. And even if you forget the experience or what you read, its effect on your model of the world persists. Your mind is like a compiled program you’ve lost the source of. It works, but you don’t know why.
The place to look for what I learned from Villehardouin’s chronicle is not what I remember from it, but my mental models of the crusades, Venice, medieval culture, siege warfare, and so on. Which doesn’t mean I couldn’t have read more attentively, but at least the harvest of reading is not so miserably small as it might seem.
This is one of those things that seem obvious in retrospect. But it was a surprise to me and presumably would be to anyone else who felt uneasy about (apparently) forgetting so much they’d read.
Realizing it does more than make you feel a little better about forgetting, though. There are specific implications.
For example, reading and experience are usually “compiled” at the time they happen, using the state of your brain at that time. The same book would get compiled differently at different points in your life. Which means it is very much worth reading important books multiple times. I always used to feel some misgivings about rereading books. I unconsciously lumped reading together with work like carpentry, where having to do something again is a sign you did it wrong the first time. Whereas now the phrase “already read” seems almost ill-formed.
Intriguingly, this implication isn’t limited to books.