 Reichenbach began his philosophical career as a neo-Kantian, a perspective that is evident in his thesis and in his first book-length effort (1920f) after his doctoral thesis, and that at least echoes in his later work. The aim of The Theory of Relativity and A Priori Knowledge is to reconcile Kantian theory in a limited way with the theory of relativity by distinguishing two senses of synthetic a priori: Principles governing the content of experience can be synthetic a priori because they are necessary, transcendental truths, or because they are non-empirical principles that form part of how we construct our representation of reality, and are thus revisable. Reichenbach endorses the latter constitutive sense of the synthetic a priori, and sees his task as scrutinizing Kant’s principles in light of the “new” reality described in the theory of relativity. Reichenbach’s—never clearly specified—process of a construction of a representation is hidden behind the idea of “coordination”. Particular principles, such as those of causality or probability, are supposed to establish a correspondence between something in experience and our representation of it in the form of a mathematically axiomatized scientific theory (à la Hilbert). What that something is, Reichenbach never clearly describes. He explicitly rejects the proposal that it is just sensation, and he admits that what he is describing is a curious case of “the coordination of two sets, of which one [...set’s] elements are first defined through the coordination” (p. 40, 1965a). While a priori in the constitutive sense, the coordination principles are contingent, they could be changed if experience makes others more convenient. The picture is very much like that C.I. Lewis offers at about the same time in Mind and the World Order (1929). 