The aim of this chapter is to provide a detailed sketch of Dewey’s theory of inquiry, which emphasizes existence as interactional and highlights the diff erence and connection between secondness and thirdness. Dewey presents the basic framework of this theory in various places throughout his oeuvre, and with each iteration it becomes more intricate. This theory is the common backbone of Dewey’s logic, his pragmatism, and his instrumentalism. It centers on his notion of refl ective experience, which is also refl ective thinking. Refl ective thinking can be diff erentiated into two types: one of these is concretely focused, the other is abstract. The diff erence between them he refers to as the logical diff erence: the diff erence between concrete and abstract logical forms. However, as aspects of the larger task of inquiry, both types are connected. Further, they are connected to a pre-refl ective and post-refl ective situation which surrounds refl ective thinking. It is here, in this non-refl ective experience, that problems emerge which induce refl ective thinking. In this theory of inquiry, knowledge is instrumental to the resolution of problems. This continuity between nonrefl ective experience, concrete refl ection and abstract refl ection is at the heart of Dewey’s pragmatism, and it is primarily informed by Peirce.