Similes and Metaphors
Now a simile, as the name imports, is a comparison of two or more things, more or less unlike in themselves, for the purpose of illustration. The thing illustrated and the thing that illustrates are, so to speak, laid alongside each other, that the less known may be made more intelligible by the light of that which is known better. Here the two parts are kept quite distinct, and a sort of parallel run between them. And the actions or the qualities of the two things stand apart, each on their own side of the parallel, those of neither being ascribed to the other.
In a metaphor, on the other hand, the two parts, instead of lying side by side, are drawn together and incorporated into one. The idea and the image, the thought and the illustration, are not kept distinct, but the idea is incarnated in the image, so that the image bears the same relation to the idea as the body does to the soul. In other words, the two parts are completely identified, their qualities interfused and interpenetrating, so that they become one. Thus a metaphor proceeds by ascribing to a given object certain actions or qualities which are not literally true of that object, and which have in reference to it only the truth of analogy.
Shakespeare: His Life, Art, and Characters, Volume I. Henry Norman Hudson, New York: Ginn and Co., 1872