 One of the most controversial claims to emerge from Kuhn’s assertions about the incommensurability of scientific theories is that the proponents of different paradigms work in different worlds (Kuhn 1962, 150; cf. Hoyningen-Huene 1990; 1993). Drawing on experiments in the psychology of perception, Kuhn argued that the rigorous training required for admittance to a paradigm conditions scientist’s reactions, expectations and beliefs (Kuhn 1962, 128; 2000 [1989], 66–71), so that learning how to apply the concepts of a theory to solve exemplary problems determines scientists’ experiences. So for example, where a proponent of the Newtonian theory sees a pendulum, an Aristotelian saw constrained free fall; where Priestley saw dephlogisticated air, Lavoisier saw oxygen; where Berthollet saw a compound that could vary in proportion, Proust saw only a physical mixture. Kuhn (and Feyerabend) used the analogy of a Gestalt switch to illustrate this point. In this way, one main source of the notion of incommensurability of scientific theories has been the development of Gestalt psychology.