 After introducing operational analysis with that refreshing discussion of the length concept, Bridgman published a long series of critical re-assessments of various fundamental physical concepts. His ruminations on length were extended into general commentary on the nature of space, and the concept of time received a similar treatment. His views on space and time were reminiscent of Henri Poincaré’s and Pierre Duhem’s: Bridgman noted that clocks had to be used in the empirical determination of the basic laws of physics, while our confidence that a clock ticks regularly was founded in the basic laws of physics governing its mechanism. Suppose we are trying to test the general theory of relativity by measuring the red-shift of light coming out of a heavy body: