In "A Defence of Poetry ," Shelley defines reason as logical thought. Reason sees what is already around it and absorbs the facts of the real world. Imagination, on the other hand, is the faculty that takes what reason sees in the everyday world and, rather than just accept...
it, perceives the higher possibilities inherent in everyday things. As Shelley puts it,
Reason respects the differences, and Imagination the similitudes of things. Reason is to Imagination as the instrument to the agent, as the body to the spirit, as the shadow to the substance.
Imagination is thus the realm of the intuitive leap, the inspiration that sees beyond the ordinary. Reason is the fodder or "instrument" that feeds the imagination, but imagination is the creative leap or "spirit" that sees the creative potential in the world.
Not surprisingly, Shelley puts imagination at the heart of poetry—poetry, he believes, transcends the ordinary, everyday world of reason and makes connections that help us see the world in new ways. Poetry is the most sublime of the art forms because it uses language, the medium most closely associated with pure thought.
For Shelley, the poetic imagination does more than create pleasure. The best and truest poetry also has a moral dimension: it excites human emotions and communicates insights that move people towards goodness. Because it relies on imagination, poetry is empathic: it allows to feel as others who are unlike us might feel.
Shelley invests poetry, because of imagination, with the highest of powers, seeing in it an expression of the divine force. Today we might consider that Shelley credits poetry with too much power, but his essay well expresses the Romantic belief in the potential of the imagination to build a better world.