 Often a real experiment that is meant to be the analogue of a thought experiment is impossible to be carried out as such due to physical, technological, ethical, or financial limitations (see, e.g., Sorensen 1992, pp. 200–202); but physical unrealizability needn’t be a defining condition of thought experiments. Rather, the main point is that we seem able to get a grip on nature just by thinking, and therein lies the great interest for philosophy. That was the position of Ernst Mach (see Mach, 1897 and 1905; for most instructive assessments of his views see Kühne 2006, pp. 165–202, and Sorensen 1992, pp. 51–75). Thought experiments are on a spectrum of different kinds of experiments. They allow us to tap into a great store of “instinctive knowledge” picked up from past experience. We will get back to Mach’s theory further down. His account of thought experiments remains one of the major theories of how thought experiments work. One of Mach’s favourite examples is due to Simon Stevin (see Mach, 1883, pp. 48–58). When a chain is draped over a double frictionless plane, as in Fig. 2a, how will it move? Add some links as in Fig. 2b. Now it is obvious. The initial setup must have been in static equilibrium. Otherwise, we would have a perpetual motion machine; and according to our experience-based “instinctive knowledge,” says Mach, this is impossible. We do not have to perform the experiment in the real world, which we could not do, anyway, since it would require a perfectly frictionless plane. Nevertheless the outcome seems compelling.