 In the physical sciences proper, such as optics and electrodynamics, the scene changes entirely and principles seem no longer to share the conventional character of the geometrical postulates. Poincaré’s “indifferent hypotheses” are those that the analyst assumes at the beginning of calculations and that are neither true nor false, but rather play roles as occupants in a structure. Indifferent hypotheses differ from geometrical conventions in one very important sense. While geometry rests on a conventional decree, the structures of physics (mechanics included) do not systematically rest on indifferent hypotheses; rather, these hypotheses have only a psychological and pedagogic function as mental constructs that are in a sense superfluous, since they can change while leaving the experimental results intact. The basis of Poincaré’s structural realism can be seen in his idea that the content is superfluous and that it is the relations expressed in natural laws that remain through time that are essential to science.