  Mach’s opposition to atomism has become one of his best known legacies, with Mach seen as an anti-realist about unobservable entities. While anti-realist arguments can indeed be found in Mach, their origins lie not in philosophical skepticism but in his bio-psychological view of science. Furthermore, his concerns about atoms were often specific to the various competing theories of the time. It should be noted that the issue of atomism was a central scientific controversy of this period; there were a variety of theories of atoms being put forward as well as varieties of alternatives. In his Principles of the Theory of Heat, Mach aligned himself with a phenomenological approach to thermodynamics—a temporarily fruitful research program that avoided the problems of positing theoretical causal entities. Mach’s experimental research was not in this area, but at stake was not just the reality of atoms but an understanding of science. Mach’s attitude toward atoms was an outgrowth of a view of science.