[Andrew Frisardi]

Welcome to the Sphere, Scott. This might be the Aristotle quote you mean. It’s from Rhetoric 1410b:

We will begin by remarking that we all naturally find it agreeable to get hold of new ideas easily: words express ideas, and therefore those words are the most agreeable that enable us to get hold of new ideas. Now strange words simply puzzle us; ordinary words convey only what we know already; it is from metaphor that we can best get hold of something fresh. When the poet calls "old age a withered stalk," he conveys a new idea, a new fact, to us by means of the general notion of bloom, which is common to both things.

I found a reference to it reading in an excellent book called Dante and Medieval Latin Traditions, by Peter Dronke, where he talks about the difference between a view of poetic metaphor as largely ornamental and a view of it as having a unique function. Dante most definitely has the latter approach:

If occasionally [Dante] uses image which amplify and adorn but add nothing essential to meaning, for the most part his imagery is consubstantial with meaning. It is used functionally, to clarify rather than to ornament; it is used in order to say things that Dante could not say in other ways. Hence we are disappointed at the many places in ancient and medieval works on rhetoric and poetics that seem to ignore this possibility. In the Latin tradition, it is commonest to discuss concepts such as imago, similitudo, metonymia, metaphora . . . in terms of ornatus. And often—though not always—this carried the implication of beautifying expression rather than of illuminating or intensifying meaning.

And Dronke quotes Dante himself on this, who wrote in one of his letters:

For we perceive many things by the intellect for which language has no terms -- a fact which Plato indicates plainly enough in his books by his employment of metaphors; for he perceived many things by the light of the intellect which his everyday language was inadequate to express.

Dronke says that Aristotle is one of the ancient writers usually said to have the “luxury” view of metaphor, although the above quote from Rhetoric suggests that Aristotle admits its fundamental and profound usefulness as well. The qualities that Aristotle says that metaphor gives or should give are:

delight
strangeness
claritas (Metaphor should clarify meaning, says A.; if it is too far-fetched, it obscures.)

About the middle one there, in Rhetoric 1404b, he writes (in Dronke’s stilted translation): “It is necessary to use alien [strange] expressions, for men wonder at newcomers, and what is wondrous is delectable.” In other words: “It is good to give everyday speech an unfamiliar air: people like what strikes them, and are struck by what is strange and unexpected.”