Letters 59.6

I do, however, find some metaphors, not, indeed, daring ones, but the kind which have stood the test of use. I find similes also; of course, if anyone forbids us to use them, maintaining that poets alone have that privilege, he has not, apparently, read any of our ancient prose writers, who had not yet learned to affect a style that should win applause. For those writers, whose eloquence was simple and directed only towards proving their case, are full of comparisons; and I think that these are necessary, not for the same reason which makes them necessary for the poets, but in order that they may serve as props to our feebleness, to bring both speaker and listener face to face with the subject under discussion.

Seneca, Letters to Lucilius 59.6, translated by Richard Mott Gummere