In the summer of 1819, between the writing of Acts III and IV of Prometheus Unbound, Shelley worked on The Cenci (R&P 236–301), a play in which he avoided, as he told his readers in the Preface, ‘the introduction of what is commonly called mere poetry’ (R&P 241). He thus reiterates his ambition of releasing poetry from its current, ‘more restricted sense’. Partly as a result of Shelley's own remarks, his tragedy has often been taken to be the ‘sad reality ’ his more idealistic poetry sublimated (R&P 237). But Shelley's interest in the transvaluations possible under poetic pressure establishes a sometimes overlooked continuity between both modes of writing. The Cenci's contribution to this debate is to stage a historical attempt to transform apparently immutable moral, religious, and political principles, rather as he will do later in ‘The Mask of Anarchy’ (R&P 301–11).

In the Preface, Shelley likens the difficulties of this task to those of ‘casuistry’. Casuistry is the bending of principles to take account of particular circumstances in individual cases, or the conflict of duties that may complicate a straightforward act of judgement. Casuistry, like sophistry, has a pejorative ring to it, although some modern philosophers have thought its flexibility exemplary – ‘the goal of ethical investigation’, G. E. Moore called it in Principia ethica. The hero of Prometheus Unbound is intended to be unambiguously good, and so to contrast with Milton's Satan, whom we are tempted by ‘a pernicious casuistry’ to exonerate on grounds of God's extreme provocation (R&P 133). In the case of the sexually abused parricide, Beatrice Cenci, Shelley is fascinated by ‘the restless and anatomizing casuistry with which men seek the justification of Beatrice, yet feel she has done what needs justification’ (R&P 240). In the culminating trial scene, Beatrice gets Marzio, one of the assassins she has suborned, to declare that she is innocent of killing the father who raped her. Is Marzio right or wrong? Is he mesmerized by Beatrice's charismatic authority, or in command of ‘a higher truth’, as he says? (V. ii. 164). Shelley implies that Beatrice's ‘dramatic character ’ forces her audience to plumb their power to grasp, in the senses of A Defence of Poetry, ‘before unapprehended relations of things’.