This quote is apparently from Piaget, but I got it from Calvin via a comment on Jeff Fleck’s blog:

intelligence is what you use when you don't know what to do, when neither innateness nor learning has prepared you for the particular situation.


The context was pretty interesting too: William H. Calvin, Pumping Up Intelligence: Abrupt Climate Jumps and the Evolution of Higher Intellectual Functions during the Ice Ages, in The Evolution of Intelligence, edited by R. J. Sternberg (Erlbaum, 2001), pp. 97-115