Listen closely to anything anyone says, and soon you'll hear analogies. We speak of time in terms of space, as like a fluid that's running out; we talk of our friends in physical terms, as in Mary and John are very close. All of our language is riddled and stitched with curious ways of portraying things as though they belonged to alien realms.
We sometimes call these metaphors, our ways to transport thoughts between the various mental realms. Some metaphors seem utterly pedestrian, as when we speak of taking steps to cause or prevent some happening. Other metaphors seem more miraculous, when unexpected images lead to astonishing insights — as when a scientist solves a problem by conceiving of a fluid as made of tubes or of a wave as an array of overlapping, expanding spheres. When such conceptions play important roles in our most productive forms of thought, we find it natural to ask, What is a metaphor? But we rarely notice how frequently we use the same techniques in ordinary thought.
What, then, is a metaphor? It might be easy to agree on functional definitions like A metaphor is that which allows us to replace one kind of thought with another. But when we ask for a structural definition of metaphor, we find no unity, only an endless variety of processes and strategies. Some are simple, as when we make an analogy by stripping away so many details that two different objects seem the same. But other forms of metaphor are as complex as can be. In the end there is little to gain by cloaking them all under the same name metaphor, because there isn't any boundary between metaphorical thought and ordinary thought. No two things or mental states ever are identical, so every psychological process must employ one means or another to induce the illusion of sameness. Every thought is to some degree a metaphor.
Once scientists like Volta and Ampere discovered how to represent electricity in terms of the pressures and flows of fluids, they could transport much of what they already knew about fluids to the domain of electricity. Good metaphors are useful because they transport uniframes, intact, from one world into another. Such cross-realm correspondences can enable us to transport entire families of problems into other realms, in which we can apply to them some already well-developed skills. However, such correspondences are hard to find since most reformulations merely transform the uniframes of one realm into disorderly accumulations in the other realm.
From where do we obtain our most productive, systematic, cross- realm correspondences? Some must be virtually born into our brains through the wiring of our paranomes; other metaphors we discover by ourselves as individuals; but most of them are learned from other members of our cultural communities. Finally, from time to time, someone discovers a new reformulation that is both so fruitful and so easy to explain that it becomes part of the general culture. Naturally, we'd like to know how the greatest metaphorical discoveries were made. But because this is buried in the past, the best and rarest of those events may never be explained at all. Our greatest ideas, like our evolutionary genes, need form only once, by accident, and then can spread from brain to brain.