Answer: Samuel Coleridge in Biographia Literaria (1817)
The thought suggested itself (to which of us I do not recollect) that a series of poems might be composed of two sorts. In the one, the incidents and agents were to be, in part at least, supernatural; and the excellence aimed at was to consist in the interesting of the affections by the dramatic truth of such emotions as would naturally accompany such situations, supposing them real. And real in this sense they have been to every human being who, from whatever source of delusion, has at any time believed himself under supernatural agency. For the second class, subjects were to be chosen from ordinary life; the characters and incidents were to be such, as will be found in every village and its vicinity, where there is a meditative and feeling mind to seek after them, or to notice them, when they present themselves.
In this idea originated the plan of the 'Lyrical Ballads'; in which it was agreed, that my endeavours should be directed to persons and characters supernatural, or at least romantic, yet so as to transfer from our inward nature a human interest and a semblance of truth sufficient to procure for these shadows of imagination that willing suspension of disbelief for the moment, which constitutes poetic faith.