The main idea of A Defence of Poetry is really a simple one. The majority of Shelley's essay lays out arguments in support of the main idea. Shelley asserts that poetry is the herald of beneficial changes in the world because poets, including the celebrated poets of Shelley's age, are the prophets and legislators of the world.
Shelley defines poetry as generated by a force unlike and even opposite to reason. He says reason synthesizes thoughts and "enumerates" their "qualities" while preserving the "differences" of their qualities. Shelley claims imagination, not reason, as the seat of poetry and says imagination analyzes thoughts and perceives the "value" of their qualities while preserving the "similitude of things," the similarity of things. Reason and imagination are thereby shown to have opposite natures and functions. He further defines poetry as music with melody and harmony:
Poetry ... produces not melody alone, but harmony, by an internal adjustment of sounds.
He then goes on to lay a case for understanding poets as the "institutors of law," the "founders of civil society," the "teachers," "prophets," and "legislators" of the world. He suggests that it is these capacities that press poetry and poets into the forefront position of receiving the "wind" of futurity as an "ever-changing wind over an Aeolian lyre" from which vantage point they are:
The most unfailing herald, ... of the awakening of a great people to work a beneficial change in opinion or institution.