A metaphor is:

"Metaphor is the juxtaposition of disparate elements of the world in which an unsuspected commonality, an illuminating partial likeness, has been discovered, and the more unlikely the juxtaposition, the greater the consequent sensation of the unifying of the world; and so the range of a writer's metaphor is a measure of the range of his cognition."

— Leon Wieseltier, in his gorgeous New York Times Book Review review of Saul Bellow:  Letters