This chapter attempts to find a measure for the “strength” of a system of field equations, which is determined by the amount of free data consistent with the system. It introduces the infinitesimal displacement field as a necessary remedy in general relativistic theory, as one can no longer form new tensors from a given tensor by simple differentiation and that in such a theory there are much fewer invariant formations. The infinitesimal displacement field replaces the inertial system inasmuch as it makes it possible to compare vectors at infinitesimally close points. After introducing these concepts, the chapter presents a discussion on relativistic field theory.