This volume collects 11 essays, five new and six reprinted, all completed between 2010 and 2020. Though the topics vary widely, each essay bears in one way or another on the description, exploration, or application of a version of naturalism called Second Philosophy. The title essay traces the source of second-philosophical thinking to the ‘natural philosophy’ of the early modern period, when ‘science’ and ‘philosophy’ weren’t separate disciplines. Those that follow explore the consequences of the second-philosophical approach for the realism/instrumentalism debate in the philosophy of science, for the challenge of radical skepticism about our knowledge of the external world, for the theory of truth and reference, and for the philosophies of logic, arithmetic, and contemporary pure mathematics. Developing these themes involves close engagement with historical figures of the 17th and 18th centuries—Locke, Hume, and Reid—and the 20th—Moore, Wittgenstein, and Austin.