The great instrument of moral good is the imagination.

Shelley’s remark comes from his 1821 essay “A Defence of Poetry,” written in response to Thomas Love Peacock’s satirical attack on poetry (“The Four Ages of Poetry,” 1820). In the essay Shelley argues that poetry is not a frivolous ornament but a civilizing force: it enlarges sympathy, refines feeling, and helps societies imagine justice beyond existing institutions. Composed during Shelley’s Italian years, amid political reaction after the French Revolution and intense debates about reform, the essay frames imagination as a moral faculty—one that enables identification with others and thus underwrites ethical progress.

Shelley treats imagination as the engine of ethical life because it lets us enter perspectives other than our own. Moral improvement, on this view, is not primarily produced by rules or calculation but by an expanded capacity for sympathy: we “feel with” others by imaginatively inhabiting their joys and sufferings. Poetry matters because it exercises and enlarges this faculty, making people more responsive to injustice and more capable of conceiving better social arrangements. The line also reflects Romantic-era psychology, where imagination is a creative, world-shaping power rather than mere fantasy—an active instrument for forming conscience and communal bonds.