First, the poet was conceived of as being definitely "possessed" by some foreign being, a god or angel, who gave utterance through his mouth and gave it only as and when it chose. Then the divine power was said to be "breathed in" to the poet, by beings such as the Muses, at special times and places, over which he had some measure of control, in that he could go himself to the places and "invoke" the Muse. Finally this "breathing in" or inspiration took on the more metaphorical sense which it has today definitely retaining, however, the original suggestion of a diminished self-consciousness. Inspiration! It was the only means, we used to be told, by which poetry could be written, and the poet himself hardly knew what it was a kind of divine wind, perhaps, which blew where it listed and might fill his sails at some odd moment after he had whistled for it all day in vain. So we were told not long ago; but today we are more inclined to think of inspiration as a mood, a mood that may come and go in the course of a morning's work.
--Owen Barfield, Poetic Diction