excerpt from A Defence of Poetry

According to one mode of regarding those two classes of mental action, which are called reason and imagination, the former may be considered as mind contemplating the relations borne by one thought to another, however produced; and the latter, as mind acting upon those thoughts so as to colour them with its own light, and composing from them, as from elements, other thoughts, each containing within itself the principle of its own integrity.

NOTES: Imagination is the superior and active faculty of the mind. Shelley divides the mental faculty into two parts: reason and imagination. Reason implies a kind of logical process that enables one to connect ideas together and/or determine their relationships to one another. It is a passive thing. Imagination, meanwhile, acts upon those thoughts. It enables creation; it is the source of our artistic desires.