This study explores the formation and evolution of Mary Shelley's philosophy of sympathy, one which she continued to revise and refine throughout her lifetime. Her novels, journals, and letters reveal a persistent desire to understand what she perceived to be a deeply fraught emotion, a moral sentiment grounded in paradox. Engaging with the Moral Sense philosophy of Anthony Ashley Cooper, the third earl of Shaftesbury (1671-1713), Francis Hutcheson (1694-1746), David Hume (1711-1776), and Adam Smith (1723-1790), Shelley insists that sympathy lies at the very heart of our ethical being, encouraging recognition of and respect for the other. Yet, as she demonstrates in her fiction-from Frankenstein; or, The Modern Prometheus (1818) to Falkner (1837)-when felt to excess, sympathy can mutate into an unnatural and harmful emotion capable of provoking antisocial, immoral, incestuous, and even suicidal behaviour. More than this, Shelley's investigation of sympathy exposes its serious limitations. Predicated on a similarity to self, sympathy, Shelley suggests, often fails when confronted with difference. Finally, through multiple perspectives, Shelley illustrates the complex and contradictory motivations behind sympathy, showing that it can arise from genuine benevolence, self-interest, or a combination of the two, an entangling of intentions that serves to further complicate this moral sentiment. Ultimately, Shelley's philosophy of sympathy acknowledges its shortcomings and potential dangers but nonetheless celebrates sympathy as a social virtue, as the locus of our moral selves.