Since I was about 9 I've been puzzled by the apparent contradiction
between being made of matter that behaves in a predictable way, and
the feeling that I could choose to do whatever I wanted. At the
time I had a self-interested motive for exploring the question. At
that age (like most succeeding ages) I was always in trouble with
the authorities, and it seemed to me that there might possibly be
some way to get out of trouble by arguing that I wasn't responsible
for my actions. I gradually lost hope of that, but the puzzle
remained: How do you reconcile being a machine made of matter with
the feeling that you're free to choose what you do?
The best way to explain the answer may be to start with a slightly
wrong version, and then fix it. The wrong version is: You can do
what you want, but you can't want what you want. Yes, you can control
what you do, but you'll do what you want, and you can't control
The reason this is mistaken is that people do sometimes change what
they want. People who don't want to want something — drug addicts,
for example — can sometimes make themselves stop wanting it. And
people who want to want something — who want to like classical
music, or broccoli — sometimes succeed.
So we modify our initial statement: You can do what you want, but
you can't want to want what you want.
That's still not quite true. It's possible to change what you want
to want. I can imagine someone saying "I decided to stop wanting
to like classical music." But we're getting closer to the truth.
It's rare for people to change what they want to want, and the more
"want to"s we add, the rarer it gets.
We can get arbitrarily close to a true statement by adding more "want
to"s in much the same way we can get arbitrarily close to 1 by adding
more 9s to a string of 9s following a decimal point. In practice
three or four "want to"s must surely be enough. It's hard even to
envision what it would mean to change what you want to want to want
to want, let alone actually do it.
So one way to express the correct answer is to use a regular
expression. You can do what you want, but there's some statement
of the form "you can't (want to)* want what you want" that's true.
Ultimately you get back to a want that you don't control.
I didn't know when I was 9 that matter might behave randomly,
but I don't think it affects the problem much. Randomness destroys
the ghost in the machine as effectively as determinism.
If you don't like using an expression, you can make the same
point using higher-order desires: There is some n such that you
don't control your nth-order desires.
Thanks to Trevor Blackwell,
Jessica Livingston, Robert Morris, and
Michael Nielsen for reading drafts of this.