In this dialogue the philosopher Plato has his characters discuss the nature of the art of the poet.  Toward the end we read this paragraph on inspiration.

For a poet is a light and winged and sacred thing, and is unable ever to compose until he has been inspired and put out of his senses, and his mind is no longer in him: every man, whilst he retains possession of that, is powerless to compose a verse or chant an oracle.  For not by art do they utter these things, but by divine influence; since, if they had fully learnt by art to speak on one kind of theme, they would know how to speak on all.  And for this reason the god takes away the mind of these men and uses them as his ministers, just as he does soothsayers and godly seers, in order that we who hear them may know that it is not they who utter these words of great price, when they are out of their wits, but that it is the god himself who speaks and addresses us through them.