Edgar Allan Poe's texts famously set reason against irrationality, usually formulated in terms of gothic horror, linguistic indeterminacy, psychosexual anxiety, racial fear, or some combination thereof. The disruptive power of chance is less noticed but nonetheless crucial to Poe's writings about the limits of reason, particularly his tales of ratiocination—“The Murders in the Rue Morgue” (1841), “The Mystery of Marie Roget” (1842–43), and “The Purloined Letter” (1844). Exploring emergent sciences of chance through his polymath investigator Dupin, Poe's use of probability theory matters in a number of registers. His tales of ratiocination confront the challenge of skepticism by working with, not against, scientific discourses of the time, suggesting that the relationship between science and romanticism is not as oppositional as it is sometimes taken to be. Concepts of chance also influence Poe's literary practice and theory, offering a framework for an aesthetic that turns out to be surprisingly realistic. Finally, as a writer attuned to popular culture, Poe both critiques and reflects everyday forms of probabilistic thinking, thus (as odd as it may sound) helping to define what it means to act rationally in mid-nineteenth-century America. By focusing on chance, we can understand Poe less as a proto-poststructuralist and more as a writer who anticipates the rise of pragmatism. It has long been recognized that Poe asks some of the hardest philosophical questions. His encounters with chance show that he has more answers than his critics have often supposed.