Abstract: Peirce's pragmatist principle is defended as a methodological rule for clarifying concepts and hypotheses. He claimed that, unlike other versions of pragmatism, his could be proved, and much of his work after 1900 is devoted to the search for a proof. The chapter discusses what sort of proof he required: it must be sufficiently strong to convince the champions of a priori metaphysics and nominalists who reject the form of realism that Peirce took to be essential to pragmatism. Later sections of the chapter discuss the resources that Peirce took to be available for constructing the proof: phenomenology, results in normative sciences like ethics and logic and semiotic.

Keywords: metaphysics, methodology, normative science, Peirce, phenomenology, pragmatism, proof, realism, semiotic,