This essay discusses four definitions of beauty from Western philosophy in light of recent experimental work from the more modern fields of psychology and biology. The first idea, derived from Plato, that beauty consists of relationships between parts, is partially confirmed by recent psychological experiments on infants and adults. The second idea, that beauty consists of one salient feature amid a mass of details, is more recent, perhaps from Hume, and is confirmed by some experiments on adults, but this finding (...) has not been replicated in non-Western cultures. The third idea, that beauty is based on utility, occurs in Plato but is more difficult to support through experiments; biology suggests that a longing for beauty, not merely for survival, is an evolutionary target. Finally, the fourth idea, that beauty is a type of cognitive pleasure, is a constant thread from Plato through the work of Aquinas and Kant and seems to confirm a preference for an optimum level of complexity by adults, but cannot explain a parallel preference for complexity in human infants. (shrink)