224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



servable. And, as an example of the absurdity to which the principle 

 leads us, it was pointed out that it would require us to deny the 

 reality of all that is past. 



Now, before coming to the main issue, it may be well to point out 

 that this example does not hold good. The past is not physically 

 unobservable, but only practically so. Every event in the past inter- 

 acted with the rest of the universe, and the effects of the interaction 

 persist in one form or another; it could, therefore, in principle be 

 deduced from an analysis of those effects, just as the course of certain 

 past events can be observed from a cinematograph film. True, 

 photography is a specially arranged type of interaction, the results 

 of which can be analyzed into a reproduction of the original events 

 with relative ease, but that is merely a practical difference. Since 

 radiation is going out from every event at every instant, and radia- 

 tion, as energy, is indestructible, there always exists the physical pos- 

 sibility of tracing it back from whatever form it may have assumed 

 to its original character, and so recovering the past. Before conclud- 

 ing that the principle is inherently absurd, therefore, it is necessary 

 to examine it with some care, for it leaves the possibility of a richer 

 universe than might at first be suspected. 



But making all due allowance for that, the criticism remains that 

 the principle does categorically deny existence to whatever cannot 

 be physically observed, and that this implies an unwarrantable as- 

 sumption of omniscience. We are, therefore, in this dilemma. If 

 we deny the principle, we have no check on idle invention; it may 

 be that all that we know, and have taken such pains to find out, is 

 trivial, while the great, important facts of the universe are not even 

 suspected and are unattainable — at least with our present human 

 faculties and knowledge, and perhaps altogether. On the other hand, 

 if we accept the principle, we close the door to all experience outside 

 that which our present knowledge allows. Our mental, like our 

 physical, universe becomes finite, and all that the future provides 

 scope for is more ingenious manipulation of things we already know. 

 And let me repeat that this dilemma is not a domestic affair for 

 physicists. In physics it concerns at present only existences observ- 

 able by sense perception, but, clearly, it is equally relevant, in the 

 appropriate forms, to all spheres of thought in which we regard our- 

 selves as apprehending some independent existence by means of 

 human faculties. 



This last sentence, I think, gives the clue to the solution of the 

 problem: "All spheres of thought in which we regard ourselves as 

 apprehending some independent existence by means of human facul- 

 ties." That is the attitude which I have assumed throughout this 

 discussion — the attitude of naive realism in which we picture an 

 objective universe existing independently of our thought of it and our 

 examination of it. It is the attitude which we always assume in 



