Recent work in the history of philosophy of science details the Kantianism of philosophers often thought opposed to one another, e.g., Hans Reichenbach, C.I. Lewis, Rudolf Carnap, and Thomas Kuhn. Historians of philosophy of science in the last two decades have been particularly interested in the Kantianism of Reichenbach, Carnap, and Kuhn, and more recently, of Lewis. While recent historical work focuses on recovering the threatened-to-be-forgotten Kantian themes of early twentieth-century philosophy of science, we should not elide the differences between the Kantian strands running throughout this work. In this paper, I disentangle a few of these strands in the work of Reichenbach and Lewis focusing especially on their theories of relativized, constitutive a priori principles in empirical knowledge. In particular, I highlight three related differences between Reichenbach and Lewis concerning their motivations in analyzing scientific knowledge and scientific practice, their differing conceptions of constitutivity, and their relativization of constitutive a priori principles. In light of these differences, I argue Lewis's Kantianism is more similar to Kuhn's Kantianism than Reichenbach's, and so might be of more contemporary relevance to social and practice-based approaches to the philosophy of science.