'That language is elevated and remote from the vulgar idiom which employs unusual words: by unusual, I mean foreign, metaphorical, extended—all, in short, that are not common words. Yet, should a poet compose his Diction entirely of such words, the result would be either an enigma or a barbarous jargon: an enigma if composed of metaphors, a barbarous jargon if composed of foreign words. For the essence of an enigma consists in putting together things apparently inconsistent and impossible, and at the same time saying nothing but what is true. Now this cannot be effected by the mere arrangement of words; by the metaphorical use of them it may.'
On Life and Poetry of Homer
, wrongly ascribed to Plutarch, Bk. I. § 16.
Poetics
, II. § 26.