From Percy Shelley’s A Defense of Poetry

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 with the human being, and perhaps within all sentient being, 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. A child at play by itself will express its delight by its voice and motions; and every inflexion of tone and every gesture will bear exact relation to a corresponding antitype in the pleasurable impressions which awakened it; it will be the reflected image of that impression; and as the lyre trembles and sounds after the wind has died away, so the child seeks, by prolonging in its voice and motions the duration of the effect, to prolong also a consciousnes of the cause. In relation to the objects which delight a child, these expressions are what poetry is to higher objects. The savage (for the savage is to ages what the child is to years) expresses the emotions produced in him by surrounding objects in a similar manner; and language and gesture, together with plastic or pictorial imitation, become the image of the combined effect of those objects, and of his apprehension of them. Man in society, with all his passions and his pleasures, next becomes the object of the passions and pleasures of man; and additional class of emotions produced an augmented treasure of expressions; and language, gesture, and the imitative arts, become at once the representation and the medium, the pencil and the picture, the chisel and the statue, the chord and the harmony. The social sympathies, or those laws from which as from its elements society results, begin to develop themselves from the moment that two human beings coexist; the future is contained within the present as the plant within the seed; and equality, diversity, unity, contrast, mutual dependence, become the principles alone capable of affording the motives according to which the will of a social being is determined to action, inasmuch as he is social; and constitute pleasure in sensation, virtue in sentiment, beauty in art, truth in reasoning, and love in the intercourse of kind. Hence men, even in the infancy of society, observe a certain order in their words and actions, distinct from that of the objects and the impressions represented by them, all expression being subject to the laws of that from which it proceeds. But let us dismiss those more general considerations which might involve an inquiry into the principles of society itself, and restrict our view to the manner in which the imagination is expressed upon its forms.