Today's Quintilian: 8.6.4-5

IV. Incipiamus igitur ab eo qui cum frequentissimus est tum longe pulcherrimus, tralatione dico, quae metaphora Graece vocatur. Quae quidem cum ita est ab ipsa nobis concessa natura ut indocti quoque ac non sentientes ea frequenter utantur, tum ita iucunda atque nitida ut in oratione quamlibet clara proprio tamen lumine eluceat. V. Neque enim vulgaris esse neque humilis nec insuavis haec recte modo adscita potest. Copiam quoque sermonis auget permutando aut mutuando quae non habet, quodque est difficillimum, praestat ne ulli rei nomen deesse videatur. Transfertur ergo nomen aut verbum ex eo loco in quo proprium est in eum in quo aut proprium deest aut tralatum proprio melius est.

4. Let us commence, however, with that species of trope, which is both the most common and by far the most beautiful, I mean that which consists in what we call translatio, and the Greeks μεταφορά (metaphora). Metaphor is not only so natural to us, that the illiterate and others often use it unconsciously, but is so pleasing and ornamental, that, in any composition, however brilliant, it will always make itself apparent by its own luster. 5. If it be but rightly managed, it can never be either vulgar, mean, or disagreeable. It increases the copiousness of a language by allowing it to borrow what it does not naturally possess; and, what is its greatest achievement, it prevents an appellation from being wanting for anything whatever. A noun or a verb is accordingly transferred, as it were, from that place in the language to which it properly belongs, to one in which there is either no proper word, or in which the metaphorical word is preferable to the proper.