Poetry demands a man with special gift for it, or else one with a touch of madness in him; the former can easily assume the required mood, and the latter may be actually beside himself with emotion.

[διὸ εὐφυοῦς ἡ ποιητική ἐστιν ἢ μανικοῦ: τούτων γὰρ οἱ μὲν εὔπλαστοι οἱ δὲ ἐκστατικοί εἰσιν.]

Aristotle (384-322 BC) Greek philosopher

Poetics [Περὶ ποιητικῆς, De Poetica], ch. 17 / 1455a.33 (c. 335 BC) [tr. Bywater (1909)]
    (Source)

Original Greek. Fyfe (below) notes μανικός to mean "genius to madness near allied," and adds "Plato held that the only excuse for a poet was that he couldn't help it." A possible source of Seneca's "touch of madness" attribution to Aristotle. Alternate translations:

Poetry implies either a happy gift of nature or a strain of madness. In the one case a man can take the mould of any character; in the other, he is lifted out of his proper self.
[tr. Butcher (1895)]

Poetry is the work for the finely constituted or the hysterical; for the hysterical are impressionable, whereas the finely constituted are liable to outbursts.
[tr. Margoliouth (1911); whiles this seems backward, Margoliouth further explains in his footnote.]

Poetry needs either a sympathetic nature or a madman, the former being impressionable and the latter inspired.
[tr. Fyfe (1932)]

Hence the poetic art belongs either to a naturally gifted person or an insane one, since those of the former sort are easily adaptable and the latter are out of their senses.
[tr. Sachs (2006)]

In order to write tragic poetry, you must be either a genius who can adapt himself to anything, or a madman who lets himself get carried away.
[tr. Kenny (2013)]