 Bridgman found that the challenges of the unknown were amply present even in very prosaic circumstances. Therefore he chose to open his discussion of operational analysis in The Logic of Modern Physics (Bridgman 1927) with the example of the most mundane of all scientific concepts: length. He was both fascinated and troubled by the fact that “essential physical limitations” forced scientists to use different measurement operations for the same concept in different domains of phenomena. Length is measured with a ruler only when we are dealing with dimensions that are comparable to our human bodies, and when the objects of measurement are moving slowly relative to the measurer. To measure, say, the distance to the moon, we need to infer it from the amount of time that light takes to travel that distance and return, and that is also the procedure taken up in Einstein’s theorizing in special relativity; “the space of astronomy is not a physical space of meter sticks, but is a space of light waves” (Bridgman 1927, 67). For even larger distances we use the unit of “light-year,” but we cannot actually use the operation of sending off a light beam to a distant speck of light in the sky and waiting for years on end until hopefully a reflected signal comes back to us (or our descendants). Much more complex reasoning and operations are required for measuring any distances beyond the solar system: