  Duhem’s famous critique of crucial experiments follows a similar pattern. Duhem argues that crucial experiments resemble false dilemmas: hypotheses in physics do not come in pairs, so that crucial experiments cannot transform one of the two into a demonstrated truth. For example, the Newtonian emission theory predicts that light travels faster in water than in air; according to the wave theory, light travels slower in water than in air. Dominique François Arago proposed a crucial experiment comparing the respective velocities. Léon Foucault then devised an apparatus to measure the speed of light in various media and found a lower velocity in water than in air. Arago and Foucault concluded for the wave theory, thinking that the experiment refuted the emission theory. Discussing Foucault’s experiment, Duhem asks whether we dare assert that no other hypothesis is imaginable and suggests that instead of light being either a simple particle or wave, it might be something else, perhaps a disturbance propagated within a dielectric medium, as theorized by Maxwell (1914, 285–89; 1954, 188–90).