 Aristotle portrays the understanding of one kind of metaphor as a stimulating exercise in analogical equation solving. Suppose Empedocles employs the term “old age” under circumstances where it looks for all the world as if what is really being discussed is the course of a single day. Old age itself lacks any immediate bearing on efforts to understand the course of a single day, so we conjecture that on this special occasion, the term “old age” stands for something with the kind of immediate bearing on efforts to understand the course of a single day that old age itself has on efforts to understand some other subject matter readily called to mind by invoking old age—namely, the course of a single human life. Just as old age constitutes the final stage of the course of a single life, what constitutes the final stage of the course of a single day is evening. Old age is to a life as evening is to a day. We thus infer that on this special occasion, “old age” is being employed to refer to evening, and we interpret the sentence in which it figures accordingly (Poetics 10 1457b).