December 2014I'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?A few months ago, as I was reading Constance Reid's excellent
biography of Hilbert, I figured out if not the answer to this
question, at least something that made me feel better about it.
She writes:

  Hilbert had no patience with mathematical lectures which filled
  the students with facts but did not teach them how to frame a
  problem and solve it. He often used to tell them that "a perfect
  formulation of a problem is already half its solution."

That has always seemed to me an important point, and I was even
more convinced of it after hearing it confirmed by Hilbert.But how had I come to believe in this idea in the first place?  A
combination of my own experience and other things I'd read.  None
of which I could at that moment remember!  And eventually I'd forget
that Hilbert had confirmed it too.  But my increased belief in the
importance of this idea would remain something I'd learned from
this book, even after I'd forgotten I'd learned it.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.  Technology
will increasingly make it possible to relive our experiences.  When
people do that today it's usually to enjoy them again (e.g. when
looking at pictures of a trip) or to find the origin of some bug in
their compiled code (e.g. when Stephen Fry succeeded in remembering
the childhood trauma that prevented him from singing).  But as
technologies for recording and playing back your life improve, it
may become common for people to relive experiences without any goal
in mind, simply to learn from them again as one might when rereading
a book.Eventually we may be able not just to play back experiences but
also to index and even edit them. So although not knowing how you
know things may seem part of being human, it may not be.
Thanks to Sam Altman, Jessica Livingston, and Robert Morris for reading 
drafts of this.