Focusing on Hegel’s engagement with Kant’s theoretical philosophy, the paper shows the merits of its characterisation as “completion”. The broader aim is to offer a fresh perspective on familiar historical arguments and on contemporary discussions of philosophical naturalism by examining the distinctive combination of idealism and naturalism that motivates the priority both authors accord to the topics of testability of philosophical claims and of the nature of the relation between philosophy and the natural science. Linking these topics is a question about how the demands of unification—imposed internally, relative to conceptions of the proper conduct of philosophical enquiry—can accommodate realism, a key element in establishing disciplinary parity between philosophy and the natural sciences. The distance that ultimately marks Kant’s and Hegel’s answers to this question justifies the interpretative claim about completion, while the conceptual patterns exemplified in the posing of the question and in their shared assumptions about its philosophical importance justifies the reconstructive claim about “idealist naturalism”.