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.

Memorable!

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