http://www.nt.armstrong.edu/METAPHOR.htm
  RHETORIC (ENGLISH 5730 U/G)

DEFINITIONS

(Gk. "carrying from one place to another")  A figure of speech in which one thing is described in terms of another.  The basic figure in poetry.  A comparison is usually implicit; whereas in simile it is explicit.  
(J. A. Cuddon, Dictionary of Literary Terms, 3rd ed., 1991)

Where simile asserts the likeness of one thing to another, metaphor asserts their identity.  Usually, though not always, something relatively abstract is identified with something relatively concrete, making it more vivid or accessible.   Since the mind seems better able to understand a new concept through concrete illustration than abstract explanation, the language is full of dead metaphors, in which illustration has been fully absorbed into concept.  Metaphor  itself (literally "a carrying over") is one, and so are "abstract" [ML, "to drag away, divert"] and explanation [L., "to smooth out, make intelligible"].   . . .  The terms commonly used nowadays for the figurative or concrete element of a metaphor and its literal or conceptual element are those suggested by I. A. Richards: vehicle and tenor.  In a good metaphor the two should have enough in common to avoid absurdity while being different enough for the vehicle to enrich the tenor as well as illustrating it.   . . .   As might be expected, creative literature uses more live metaphors than ordinary language does.  These are distinguished from dead metaphors by the the fact that the figurative element has not been absorbed into the conceptual, but it should be emphasized that the specific images conveyed (which may differ somewhat from reader to reader anyway) are less important than the vehicle's ability to make the idea in the tenor powerfully connotative.
(Ian Ousby, ed., The Cambridge Guide to Literature in English, 1988)

An analogy identifying one object with another and ascribing to the first object one or more of the qualities of the second.  . . .  The tenor is the idea being expressed or the subject of the comparison; the vehicle is the image by which this idea is conveyed or the subject communicated.  . . .  Aristotle praised the metaphor as "the greatest thing by far" for poets--a sentiment seconded by Ezra Pound, who endorsed Aristotle's calling apt metaphor "the hallmark of genius"--and saw it as the product of their insight, which permitted them to find the similarities in seemingly dissimilar things.  . . .   According to a fairly ingenuous notion of language, abstractions can be treated only in terms that are not abstract, presumably because the primitive mind cannot handle abstractions.  But no evidence establishes the existence of any such limitations.   To presume that any human being has to have a grasp of physical "pulling away" before being able to grasp an abstract "abstraction" is little more than bigotry.
(Harmon & Holman, A Handbook to Literature, 2000)

Metaphors are necessary not only to our conceptual structure of everyday experience, but also to the structure of theories across academic disciplines.   Theorists often begin with analogy, a perceived resemblance between a thing as yet not identified and another known, familiar thing.  Researchers then pursue the analogy to see how far the resemblances between two phenomena can be extended.  This pursuit is a primary way theories are tested, advanced, or replaced by others.  Of the sciences, Kenneth Burke says, "Whole works of scientific research, even entire schools, are hardly more than the patient repetition, in all its ramifications, of a fertile metaphor."  Two examples from physics are black holes and chaos, both of which are metaphors that have seized the imagination of theorists in other domains of knowledge, such as business, political science, and literature, and have influenced the direction of study in these secondary fields.  The first phrase, black hole, is, in fact, now so popularized that it is even used metaphorically to express fear of our fate and that of the universe, or to explain the sudden and complete disappearance of anything lost--from a dream to a weekend to a book of stamps.
(Dona J. Hickey, Figures of Thought for College Writers, 1999)

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