 As the nineteenth century came to a close, critics of the Back to Kant movement, including Ernst Mach, began to question Helmholtz’s and Lange’s appeal to the thing in itself. Edgar (2012) identifies a “deep tension”: Helmholtz and Lange “wanted to conceive of things in themselves as the causes of our sensations, while their own accounts of causal inference ruled that claim out” (p. 1). Edgar (2012) goes on to argue that “These Neo-Kantians’ problems with things in themselves were fully solved only by Ernst Mach, in the context of the uncompromisingly austere theory of knowledge he developed in the early 1880s, in his 1882 ‘The Economical Nature of Physical Inquiry’ and his 1884 ‘Contributions to the Analysis of Sensations’” (p. 2). While Mach is critical of Helmholtz and of Lange, Edgar emphasizes nonetheless that: