At any rate, the point we wish to stress is that while it is correct to say that the objective world is a synthesis of private views or perceptions, we should not forget to add that the private views are themselves the product of a synthesis of sensations. But inasmuch as science must deal with the general, and not merely with the particular, and inasmuch as it is the common objective world that renders this general knowledge possible, it will be this world that the scientist will identify with the world of reality. Henceforth the private views, though just as real, will be treated as its perspectives. We must recognise, therefore, that the common objective world, whether such a thing exists or is a mere convenient fiction, is indispensable to science, if in no other capacity than as a working hypothesis. And neo-realism, as we have said, when pursued consistently, renders a collective view impossible. Furthermore, from the standpoint of science, whether it be classical physics, relativity, physiology or psychology, neo-realism is untenable, for under no circumstances can vision and our knowledge of nature in general be considered a matter of direct apprehension. In much the same way, we know that 7 times 8 is 56, and we obtain this result immediately, without reflection and without having to count on our fingers. Were it not for the fact that the neo-realist recollects the difficulty he experienced in learning the multiplication table, he would presumably be telling us that our knowledge of the properties of numbers was also a matter of direct recognition. From the standpoint of science, there is very little difference between the alleged immediacy of perception and our knowledge of the product of 7 times 8. Neither of these two forms of knowledge can be viewed in the light of direct recognitions. At any rate, regardless of our attitude towards neo-realism, in scientific discussions we must define the meaning of our words clearly and cease to apply the same word, “simultaneity” to a simultaneity in the scientific sense, as holding for the objective world, and to a simultaneity in the neo-realistic sense, as assumed to be apprehended directly. It is remarkable that Dr. Whitehead, who has coined such a profusion of new words when the necessity was scarcely apparent, should have missed this splendid opportunity when it happened to be imperative.[58]
However, it is not our intention to discuss neo-realism further, for the doctrine is of no interest to science. All we wish to point out is that in any scientific theory, whether it be due to a Newton, a Maxwell or an Einstein, we can make up our minds from the start that the author will deal with the objective world of science, treating the private views as mere perspectives, and will frame his concepts and definitions accordingly. It would appear, then, that if Dr. Whitehead wishes to defend a doctrine which stands in conflict with the views of every scientist from Newton to Einstein, it would have been conducive to greater clarity had he restricted his attacks to classical science. For when we attack a new theory such as Einstein’s, which has introduced such a wealth of revolutionary ideas into our understanding of nature, it is to be presumed that the reasons for our dissent arise from those ideas and not from the classical ones which the theory has left intact. Thus, if it were to be proved that matter was continuous and not atomic, we should not hold Einstein’s theory responsible for our past belief in the atomicity of matter. Rather would we challenge the experiments and theories of those classical physicists who thought they had established atomicity. On the other hand, if we objected to the view that the simultaneity of external events was relative to the observer’s motion, claiming it to be absolute, then of course we should be justified in criticising Einstein’s theory proper, since the relativity of simultaneity is peculiar to it.
To be sure, Dr. Whitehead appears to be under the impression that he is merely attacking the new revolutionary ideas introduced by Einstein. Thus, he tells us that he is merely defending “the old-fashioned belief in the fundamental character of simultaneity,” but one may well wonder to what period of human thought he is referring. Classical science, some three hundred years ago, had recognised the error of this confusion between external events and the instants at which they were perceived, precisely because this attitude was found to lead to results in utter conflict with experience.
As Weyl expresses it, referring to Römer’s discovery in 1675: “The discovery that light is propagated with a finite velocity gave the deathblow to the natural view that things exist simultaneously with their perception.” Needless to say, that which applies to luminous transmissions applies in exactly the same measure to the propagation of nerve excitation through the body and to all other species of transmissions.
The general conclusions we have reached from a discussion of simultaneity in different places apply without great change when we consider a number of other concepts which in a dim way we half feel and half comprehend. Thus, consider the case of temperature. Why does the scientist have recourse to thermometers rather than trust to the immediate disclosures of his senses? Merely because the thermometer furnishes an objective definition of the equality of two temperatures, and because it can discern differences which our senses would be unable to detect. The result is that when his thermometer indicates differences in temperature of several hundred degrees centigrade—say, temperatures of -200° and +400°—he maintains that such a difference exists, even though to our senses a scalding sensation would result in either case. But, however imprecise and unreliable our direct appreciation of temperature may be, our direct recognition of a simultaneity of spatially separated events is vaguer still. For whereas men may have no great difficulty in agreeing that one room is warmer than another, even without the aid of a thermometer, they will express the most conflicting opinions when asked which of two explosions occurred first. Their answers, unless governed by the results of physical measurements, will vary with their relative positions.
Now, because the scientist measures temperature by the thermometer, no one would accuse him of wishing to imply that the concept of temperature would cease to exist were his thermometer to fall to the ground and be broken. Why, then, should the fact that we determine simultaneity in different places by means of light signals imply that we are making the concept of simultaneity dependent on the behaviour of light, and that simultaneity could have no meaning for blind people?
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.”
Inasmuch as these arguments are advanced in a spirit of criticism, we must presume that according to Dr. Whitehead, had transmissions other than optical ones been considered, determinations differing from Einstein’s would have ensued.[59] But a contention of this sort would be fundamentally untrue. Einstein’s determinations would have yielded exactly the same results had sound transmissions been substituted for optical ones; and the only reason optical ones are continually referred to is because they permit observations of greater accuracy. We shall now consider the reasons for these statements.