independently of a theoretical system in which they feature.8 Suppose there is a dispute about a theoretical hypothesis H. Ideally, there must be some prediction "of an experimental fact" drawn from H. The experiment is then performed and if the "fact is not produced", the hypothesis H "will be irrevocably condemned" (1894, 82). Duhem shows that this account of testing is an illusion, since no hypothesis taken in isolation from a theoretical system implies any predictions. As he put it: "The prediction of the phenomenon whose nonproduction will cut off the debate does not derive from the disputed proposition taken in isolation but from the disputed proposition joined to this whole group of theories". Hence, when the prediction is not brought out "it is the whole theoretical scaffolding used by the physicists" which is "shown to be wanting" (1894, 82). But where does the error lie? The experiment cannot pinpoint the culprit among the parts of the whole theoretical system. Duhem's conclusion is that how the blame is distributed among these parts is not a matter of "logical necessity" (1894, 83). All this is rather well-known. What is not typically perceived is that Duhem uses this kind of argument to show that explanatory hypotheses do not have empirical content of their own. Immediately after the logical argument, he offers an illustration by means of Newton's emission theory of light, which he took it to be a typical case of an explanatory theory. On this theory, light is formed of very small particles emitted with great velocities by light sources. These particles (the projectiles, as Duhem calls them) are subjected to distancedependent attractive and repulsive forces and permeate all bodies. This set of "essential hypotheses", linked (and only linked) with many others entail that light travels faster in water than in the air. This prediction, noted by François Arago, was tested by Léon Foucault in a famous experiment. Duhem was quick to point out that the negative result of the experiment (viz., that light tr