214 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 193 8 



unobservable, we confine the universe within the bounds of human 

 potentialities and make nonsense of history. What are we to do? 



It is evident that we must begin by examining the word "unobserv- 

 able." Both sides wish to exclude the binkum; both wish to allow the 

 universe a richer content than we can at present perceive. The "un- 

 observable" that is to be proscribed must therefore include the obvi- 

 ously idle fancies but exclude the legitimately transcendental, and we 

 must define it so that it does so inevitably, without special pleading. 

 I need scarcely add that we must distinguish between "unobservable" 

 and "unobserved" — between what we cannot possibly observe and 

 what we have not in fact observed. This needs no elaboration; we 

 agree that an entity is not to be rejected merely because we have not 

 observed it. But what do we mean by "possibly" observable? 



There are reasons of various kinds why we may be prevented from 

 observing a thing, but I think they can all be summarized under four 

 headings. First of all, there is what I will call the practically un- 

 observable — i. e., that which is unobservable because of the practical 

 difficulties of observing it. The far side of the moon affords an ex- 

 ample. That region is unobservable because we have not solved the 

 problem of interplanetary travel, or succeeded in observing the moon 

 reflected in Mars, or learnt so much about the properties of matter 

 that we are able to deduce from observations of the near side of the 

 moon what the configuration of the far side must be, or performed 

 some other feat of practical ingenuity. It is conceivable that in time 

 this disability will be removed, so that practical unobservability may 

 be merely a temporary characteristic. 



Secondly, there is the humanly unobservable; by which 1 mean the 

 unobservable which is so because we do not possess the necessary 

 faculties for observing it. I cannot, of course, by the nature of the 

 case, give examples of this, but I can give analogies. A great deal of 

 the universe would be humanly unobservable if we had no sense of 

 sight; and to the musically insensitive the significance of a great 

 composer may be said to be humanly unobservable. If, then, there 

 is in the universe some existence which no creature has the faculty of 

 apprehending, that existence is humanly unobservable. 



Thirdly, there is what I will call the physically unobservable. A 

 thing may be said to be physically unobservable when we have the 

 faculty for observing it if nature will cooperate, but nature gives that 

 faculty no opportunity for exercise. Thus, if somewhere in space 

 there occurred an event from which no signal — light ray or sound 

 wave or anything else — proceeded to other places, and if there were 

 repulsive forces which prevented us from ever reaching the place of 

 occurrence, that eveDt would be physically unobservable. We have 

 eyes to see it, and hands to touch the bodies concerned in it, but we 

 cannot use them for that purpose. An example important in physical 



