The physicist Ernst Mach in his 1883 The Science of Mechanics described an experiment that showed how experience is supplemented by ideas, what he called the “economy of science”. In the experiment he described how an elastic rod fixed in a vice can be made to show slow vibrations, which can be seen and felt. If the rod is shortened, the vibrations will increase and can be felt but cannot be seen, and if shortened again, the rod will emit a highpitched sound but the vibrations cannot be seen or felt. In each case, whether or not they are seen, the conception of “vibrations” can be retained, it can be deployed as “serviceable”, or what Mach called “economical”, to explain the phenomena (Mach 1883: 587-588). This is not, according to Mach, a great scientific discovery but rather something we learn as a child-we supplement from previous experience what we cannot directly experience, such as a moving object going out of view behind a pillar and returning in view as it continues its motion. We can supplement unseen motion as continuing even though it is not experienced. As Mach concludes: “We fill out the gaps in experience by the ideas that experience suggests”.