The solving of a problem is an aesthetic experience. It involves a reward- mechanism going beyond the practical outcome of the solution and the sense of achievement in the eyes of others — for an unsolved intellectual problem, explicit,as in mathematics, or inexplicit, as a trick recurrence of pattern in wallpaper, rankles as a state of physical unease. This experience appears to be common to scientists and to chimpanzees, and must have played an important adaptive part in fostering the restlessly exploratory behaviour of primates.