 The first is what we may call explanatory empiricism, which denies the validity of inferences from observable phenomena to causes that cannot themselves be observed. Perhaps the best known modern example is that of the physicist and philosopher Ernst Mach (1838–1916). At least for most of his life, Mach rejected the theory of atoms as an example of “hypothetico-fictive physics” (Mach 1992: 136) and would respond to believers in atoms with the words, Habn S’eins gsehn? (Have you seen one?) (Meyer 1992: 151). (For a slightly different view of Mach’s atomism, see Banks 2003: 12–14.) A more recent example is the “constructive empiricism” of Bas van Fraassen, which holds that the aim of science is to produce theories that are empirically adequate, a theory being empirically adequate if “what it says about the observable things and events in this world, is true” (van Fraassen 1980: 12). This view also rules out inferences to unobservable entities (van Fraassen 1980: 72). While this may sound like a modern position, it was held by the first people to call themselves “empiricists,” namely a group of ancient medical writers, who rejected inferences based on reasoning rather than the observed association of phenomena (sect. 3.3).  