This chapter introduces Active Semiotic Externalism, based on the central thesis that cognition is semiosis—a process that is active, embodied, situated, and distributed. Semiosis is modelled as an irreducibly triadic relation (Sign-Object-Interpretant), on the understanding that relations are explanatorily more fundamental than entities. Peirce’s categories (Firstness, Secondness, Thirdness) serve as a conceptual tool for systematically analyzing the internal constitution, the mechanisms, and the morphological variety of triadic relations. In Active Semiotic Externalism, to understand cognition is to understand how the regularity of sign action is distributed across time.