Source identified1
“The great instrument of moral good is the imagination; and poetry administers to the effect by acting upon the cause.”
Shelley treats imagination as the capacity that lets a person move beyond a narrow private viewpoint and enter the pains and pleasures of others, with poetry strengthening that sympathetic faculty.
Source note: A Defence of Poetry, written in 1821 and first published posthumously in 1840. The represented wording is verified in Project Gutenberg #5428 and the standalone Wikisource transcription; the immediately preceding sentences define moral imagination through identification with other people rather than through fantasy alone.