Today's Quintilian: 8.6.6-7

VI. Id facimus aut quia necesse est aut quia significantius est aut, ut dixi, quia decentius. Vbi nihil horum praestabit quod transferetur, inproprium 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 adfectibus nomen. VII. 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".

6. This change we make, either because it is necessary, or because it adds to significance, or, as I said, because it is more ornamental. Where the transference produces no one of these effects, it will be vicious. From necessity the rustics speak of the gemma, "bud," of the vines (for how else could they express themselves?) and say that the corn thirsts and that the crops suffer. From necessity we say that a man is hard or rough because there is no proper term for us to give to these dispositions of the mind. 7. But we say that a man is inflamed with anger, burning with desire, and has fallen into error, with a view to significance or force of expression, for none of these phrases would be more significant in its own words than in those adopted metaphorically. The expressions, luminousness of language, illustrious birth, storms of public assemblies, thunderbolts of eloquence, are used merely for ornament; and it is thus that Cicero calls Clodius in one place a source, and in another a harvest and foundation, of glory to Milo.