Enjoy opaque writing? This is Galvano della Volpe's account of metaphor from Critique of Taste, initially citing Aristotle (parentheses and references omitted):
"In Rhetoric we are told that metaphor provides us with easy instruction and knowledge "through the genus" - in other words in so far as it is a general notion or idea. The same conception is repeated today, after Castelvestro and others by I.A. Richards when he summarizes the principle of metaphor as "a combination of general aspects". Again, in the Poetics Aristotle says that "the right use of metaphor means an eye for resemblances", that is, that "metaphors should be drawn from objects which are proper to the object, but not too obvious; just as, for instance, in philosophy it needs sagacity to grasp the similarity in things that are apart". Given these premises, Aristotle concludes by showing under the heading of Logic in the Topica that similarity in metaphor, "for those who use metaphors always do so on account of some similarity" is the same categorical norm as the similarity or sameness which regulates inductive, hypothetical and definitional reasonings. To limit ourselves to the latter, we are told that: "(The consideration of similarity) [=sameness] is useful for the assignment of definitions because, if we can see what is identical in each particular case, we shall have no doubt about the genus in which we must place the subject under discussion when we are defining it; for, of the common predicates, that which falls most definitely in the category of essence must be the genus"."
Two things are abstracted to the degree they constitute equivalents; this also means: such that each possesses a function common to both. Then potentially each can be decomposed into parts contrasting with or consonant with this function.