 Ernst Mach (1838–1916) occupies a central position in the history of neutral monism. He influenced William James and Bertrand Russell and, through them, all of the writers on neutral monism in the English speaking world. His importance for the development in the German speaking world is hard to overestimate. Among the philosophers to build on Mach’s ideas was Rudolf Carnap in his Aufbau (1928). As a physicist who also did physiological and psychological research, Mach strove to adopt an inclusive and economical framework that would allow him to pursue all of these inquiries in a unified and coherent fashion. In the simple components of experience—hot and cold, red and green, sweet and sour, etc.—he finds typical examples of a much larger group of elements whose functional interrelations are studied by the various sciences (see Banks 2014, ch. 1 on the resolute realism of Mach's view). While a given element is, intrinsically, neither mental nor physical, the various groups to which it belongs may display functional relationships that are characteristic of physics or of psychology. In this case the neutral element forms part of the subject matter of physics and of psychology. In the following quotation Mach uses a color and our perception of it to illustrate this point. A single neutral element—the color—gets to be both the physical color of a physical object and our mental perception/sensation of it. The color can be called physical, qua constituent of the one group, and mental (a sensation), qua constituent of the other group, but is the same unchanging and intrinsically neutral element that figures in these two different contexts: