Incipiamus igitur ab eo qui cum frequentissimus est tum longe pulcherrimus, translatione dico, quae μεταφορὰ 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. neque enim vulgaris esse neque humilis nec insuavis apte ac 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. id facimus aut quia necesse est aut quia significantius est aut, ut dixi, quia decentius. ubi nihil horum praestabit quod transferetur, improprium erit. necessitate rustici gemmam in vitibus (quid enim dicerent aliud?) et sitire segetes et fructus laborare; necessitate nos durum hominem aut asperum: non enim proprium erat quod daremus his affectibus nomen. iam incensum ira et inflammatum cupiditate et lapsum errore significandi gratia; nihil enim horum suis verbis quam his arcessitis magis proprium erit. illa ad ornatum, lumen orationis et generis claritatem et contionum procellas et eloquentiae fulmina, ut Cicero pro Milone Clodium fontem gloriae eius vocat et alio loco segetem ac materiem.
(Quintilian, Inst. Or. 8.6.4-7)
Let us begin, then, with the commonest and by far the most beautiful of tropes, namely, metaphor, the Greek term for our translatio. It is not merely so natural a turn of speech that it is often employed unconsciously or by uneducated persons, but it is in itself so attractive and elegant that however distinguished the language in which it is embedded it shines forth with a light that is all its own. For if it be correctly and appropriately applied, it is quite impossible for its effect to be commonplace, mean or unpleasing. It adds to the copiousness of language by the interchange of words and by borrowing, and finally succeeds in accomplishing the supremely difficult task of providing a name for everything. A noun or a verb is transferred from the place to which it properly belongs to another where there is either no literal term or the transferred is better than the literal. We do this either because it is necessary or to make our meaning clearer or, as I have already said, to produce a decorative effect. When it secures none of these results, our metaphor will be out of place. As an example of a necessary metaphor I may quote the following usages in vogue with peasants when they call a vinebud gemma, a gem (what other term is there which they could use?), or speak of the crops being thirsty or the fruit suffering. For the same reason we speak of a hard or rough man, there being no literal term for these temperaments. On the other hand, when we say that a man is kindled to anger or on fire with greed or that he has fallen into error, we do so to enhance our meaning. For none of these things can be more literally described in its own words than in those which we import from elsewhere. But it is a purely ornamental metaphor when we speak of brilliance of style, splendour of birth, tempestuous public assemblies, thunderbolts of eloquence, to which I may add the phrase employed by Cicero in his defence of Milo where he speaks of Clodius as the fountain, and in another place as the fertile field and material of his client’s glory. (tr. Harold Edgeworth Butler)