  A principle of historical continuity is invoked in the conclusion of Duhem’s primary work on Maxwell. Duhem evaluates there an interpretation of Maxwell’s work he attributes to Heaviside, Hertz, and Cohn, among others. He quotes Hertz as stating that: “what is essential in Maxwell’s theories is Maxwell’s equations.” He takes this to be Hertz’s way of salvaging what is valuable in Maxwell from the midst of logical errors and incoherence, which are not only difficult to correct, but which have frustrated many illustrious mathematicians. But Duhem cannot accept Hertz’s implied criterion of identity for physical theories. He asserts that he might accept such a criterion for algebra but “a physicist is not an algebraist”: