Charles Peirce’s Illustrations of the Logic of Science is an early work in the philosophy of science and the official birthplace of pragmatism. It contains Peirce’s two most influential papers: “The Fixation of Belief” and “How to Make Our Ideas Clear,” as well as discussions on the theory of probability, the ground of induction, the relation between science and religion, and the logic of abduction. Unsatisfied with the result and driven by a constant, almost feverish urge to improve his work, Peirce spent considerable time and effort revising these papers. After the turn of the century these efforts gained significant momentum when Peirce sought to establish his role in the development of pragmatism while distancing himself from the more popular versions that had become current. The present edition brings together the original series as it appeared in Popular Science Monthly and a selection of Peirce’s later revisions, many of which remained hidden in the mass of messy manuscripts that were left behind after his death in 1914.