In March 1818 Shelley set off for Italy never again to return to England. Thereafter, his life was constitutionally unsettled, a patchwork of moves to and removals from houses in Pisa, Lucca, Livorno, Rome, Naples, Venice, and elsewhere. He began writing Prometheus Unbound (R&P 130–210) in a summer house at Este. He continued composing it the following spring, writing Acts II and III among the ruins of the Roman baths of Caracalla. The picture of a Romantic poet alone with his vision amid the relics of a glorious past is irresistibly Romantic. Shelley's own attitude towards the poem was indulgent. He frequently called it his ‘favourite’, written for the few or the ‘elect’ like Laon and Cythna. The death of Clara Shelley, aged 1, after his mismanagement of medication and travel, and his consequent estrangement from Mary, would have enforced his isolation. Act IV was written after the second hammer-blow of the death of the 3½ - year-old William Shelley. Perhaps such misery might also have stimulated a compensatory extremism of creativity. He told Peacock in April 1819 that he had now finished ‘a drama, with characters & mechanism of a kind yet unattempted’ (Letters, ii. 94). Echoing Milton, Shelley used his classical model, Aeschylus’ play Prometheus Bound, as a foil for his fiercely revisionist effort.

By the time he writes its ‘Preface’, though, Shelley is busy assimilating his lyrical drama to a consistent poetic practice. Other shorter poems of the same time, like the fragmentary ‘A Vision of the Sea’ (H&M 596–600), harbour more directly therapeutic content; impossibly, ‘A Vision’ sustains a final image of mother and child above a shipwreck before itself falling prematurely silent, expressively deferring, perhaps, to Mary's own withdrawal. Prometheus Unbound, though, is consistent with Shelley's ‘purpose … hitherto … simply to familiarise the highly refined imagination of the more select classes of poetical readers with beautiful idealisms of moral excellence’ (R&P 135). Shelley's Prometheus, unlike the escapee of Aeschylus’ own lost Prometheus Unbound, is ‘the type of the highest perfection of moral and intellectual nature’ and reaches no pragmatic accommodation with his oppressor, Jupiter (R&P 133). But the most basic difficulty for Shelley's reader is less the learnedness of allusion than the hierarchy of characters.