Poetry, in a general sense, may be defined to be "the expression of the imagination": and poetry is connate with the origin of man. Man is an instrument over which a series of external and internal impressions are driven, like the alternations of an ever-changing wind over an Aeolian lyre, which move it by their motion to ever-changing melody. But there is a principle within the human being, and perhaps within all sentient beings, which acts otherwise than in the lyre, and produces not melody, alone, but harmony, by an internal adjustment of the sounds or motions thus excited to the impressions which excite them. It is as if the lyre could accommodate its chords to the motions of that which strikes them, in a determined proportion of sound; even as the musician can accommodate his voice to the sound of the lyre.
Percy Bysshe Shelley, Excerpt from “A Defence of Poetry,” written 1821, published 1840.
Greek Modes On A 15-String Lyre (Link)
Upper image: Brygos Painter, Rhyton in form of dog's head showing tortoiseshell lyre, 480-470 BC, Musée départemental d'archéologie Jérôme Carcopino, Aléria, Haute-Corse.
Lower image: Fair copy of "A Defence of Poetry."