In his essay, ‘A Defense of Poetry’, Shelley’s final claim that “Poets are the unacknowledged legislators of the world” has its basis in his opening view of the human mind.
For him the human mind performs two very important and very different “mental actions”: it reasons and it imagines. Shelly argues Reason is “contemplating the relation of one thought to another” but that Imagination is the “production of thoughts”; in effect imagination is the “synthesis”, or interpretation of values and forms from “existence itself”, while Reason is the analysis of those values, and concerns itself only in the relations of values of forms. For example: Imagination would look at existence itself and synthesize the value and concept of centimeters, while Reason would deal only with the analysis of centimeters once the concept is thought; in essence, reason analyzes and measures the values produced by the imagination.
So when Shelley argues “Poetry is the expression of the imagination and connate with the origin of man” he is saying Poetry is the production of human values and concepts and so by producing human values is laying down the values and laws the world lives by: in essence Poetry is the legislation of the world.
Shelley’s point that Poetry is the refined expression and “reflected image” of the stimuli that inspires and excites it, enforces the idea that Poetry is not only the creation of human values but also allows them to evolve and develop: because when it is “man in society, with all his passions and pleasures” that next becomes the subject of poetry and “Language, colour, form, and religious and civil habits of action are all the instruments and materials of poetry” then Poetry, as “the order from which the highest delights result” is the perfection of our expression of humanity and human values.
Poets are, “in the most universal sense” men especially gifted in observing the order that “most approximates to beauty” are so are “those who imagine and express this indestructible order”. For Shelley, Poets are the ones who “in the infancy of the world” developed the “social sympathies, or those elemental laws from which society develops” by observing and expressing the “highest order” to produce the society we live in. Poets “are not only the authors of language and of music, of the dance and architecture and statuary and painting; they are also the institutors of laws and the founders of civil society”.
In Conclusion, Poets by “not only beholding intensely the present as it is, and discovering those laws according to which present things ought to be ordered, but also beholding the future in the present” are the ones who shape and develop society. In effect Shelley is arguing Poets are anyone who can imagine a better order for things and try to express it; they are the revolutionaries, the innovators, and “legislators of the world”.
Peacock, Thomas Love. The four ages of Poetry; Shelley, Percey. A defence of Poetry. 1820.