Analogy of proportion is the basis of the figure of speech known as metaphor. It would be a mistake, however, to infer from this that what is thus analogically predicated of a number of things belongs intrinsically and properly only to one of them, being transferred by a mere extrinsic denomination to the others; and that therefore it does not express any genuine knowledge on our part about the nature of these other things. It does give us real knowledge about them. Metaphor is not equivocation; but perhaps more usually it is understood not to give us real knowledge because it is understood to be based on resemblances that are merely fanciful, not real. Still, no matter how slender and remote be the proportional resemblance on which the analogical use of language is based, in so far forth as it has such a real basis it gives us real insight into the nature of the analogues. And if we hesitate to describe such a use of language as "metaphorical," this is only because "metaphor" perhaps too commonly connotes a certain transferred and improper extension of the meaning of terms, based upon a purely fanciful resemblance.
Peter Coffey, Ontology, pp. 38-39.