Simile and Metaphor differ only in degree of stylistic refinement. The Simile, in which a comparison is made directly between two objects, belongs to an earlier stage of literary expression; it is the deliberate elaboration of a correspondence, often pursued for its own sake. But a Metaphor is the swift illumination of an equivalence. Two images, or an idea and an image, stand equal and opposite; clash together and respond significantly, surprising the reader with a sudden light.
—Sir Herbert Read English Prose Style (1928)