A letter of more than 2700 words from Frederick II, King of Prussia to Voltaire discussing his literary works and philosophical ideas. Frederick opens the letter by expressing his concerns about Voltaire’s frequent ill health. He has consulted doctors who have provided a long-range diagnosis, and he asks for a full statement of Voltaire’s symptoms. He then complains that Voltaire has added no criticised the poems he sent, and encloses revisions of two poems in response to Voltaire’s critiques, with a self-deprecating note on his incurable ‘mania’ for poetry, and an expression of admiration for Voltaire’s genius in rising above his physical weakness. Frederick expresses admiration for Voltaire’s ‘Épitre sur l’Envie’ and for his tragedy Mérope, offering his views on the role of love in modern theatre, and adding a verse encomium on the diversity of Voltaire’s achievements from history to science (citing his elucidation of Newton) to tragedy. Turning from literature to philosophy, the heart of the letter is a detailed consideration of some of the defining metaphysical debates of the Age of Enlightenment. Frederick expresses his scepticism at the arguments in favour of determinism, but declares himself concinced of the principle of ‘sufficient reason’ (that a rational cause lies behind every effect, and that in this sense ‘chance is the synonym of nothing’. He therefore argues that the true determinant of human actions is reason itself, and describes the idea of the deity which he deduces from this. In the last section of the letter, Frederick expresses mock-scepticism at the reported admiration for him of the Marquise du Châtelet, mentioning that he has discussed it with Leibniz, and joking that he now considers himself a rival of Newton’s for her favours.