From Gregory Bateson: Steps to an Ecology of Mind. p. 26:
After he tells his daughter he uses about a quarter of his brain:
“Daughter: Daddy why don’t you use the other three-quarters of your brain?
“Father: Oh, yes–you see the trouble is that I had school-teachers too. And they filled up about a quarter of my brain with fog. And then I read newspapers and listened to what other people said, and that filled up another quarter with fog.
“Daughter: And the other quarter, Daddy?
“Father: Oh–that’s fog I made for myself when I was trying to think.”