The sole purpose of all these measurements is to render precise and to clarify concepts which we understand only in a dim, indefinite way. In the majority of cases these concepts have arisen from crude experience. There are instances, however, where they have arisen in a more sophisticated manner. Suppose, for example, that our bodies were insensible to heat or cold. The behaviour of our thermometers would still suggest an abstract concept which we might call “temperature.” In fact, even were no thermometers at our disposal, the concept of temperature would be introduced eventually into physics for theoretical reasons. “Entropy” affords us an illustration of a concept introduced in this way. Our senses cannot detect it and our instruments do not record it directly. Nevertheless, it has imposed itself on science. A difference in entropy can be measured indirectly, just as temperature, duration and space can be measured, but always by physical means; and this is indeed the only way we can increase our understanding of these concepts, and agree on what we are talking about. Incidentally, we may recall that it was through the medium of physical measurements that Einstein was able to demonstrate the relativity of simultaneity, which our so-called direct apprehension of nature had never even led us to suspect. Hence, to accept the relativity of simultaneity, which issues from highly sophisticated physical measurements, and then to cling to the theory of a direct apprehension of nature, is a course which, to any logical mind, must appear to be the height of inconsistency. We have now to consider a second criticism also advanced by Dr. Whitehead. In it he appears to have abandoned momentarily the doctrine of direct apprehension, recognising (presumably for the sake of argument) that a determination of simultaneity at spatially separated points may require the introduction of physical propagations, but he objects to Einstein’s exclusive appeal to optical transmissions for this purpose. Thus, he tells us: “Also there are other physical messages from place to place; there is the transmission of material bodies, the transmission of sound, the transmission of waves and ripples on the surface of water, the transmission of nerve excitation through the body, and innumerable other forms which enter into habitual experience. The transmission of light is only one from among many.” [Pg 180]