SCIENCE AND THE UNOBSERVABLE — DINGLE 215 



history is the possibility of observing absolute motion. We have, in 

 the Michelson-Morley experiment and similar devices, instruments 

 which are able to detect absolute motion if nature will provide them 

 with the necessary messengers, but this she fails to do. No matter 

 how we may be changing our motion with respect to other bodies, our 

 absolute motion always appears to be nonexistent. (I am assuming, 

 without prejudice, for the sake of illustration, that Prof. Dayton 

 Miller's experiments do not contradict this statement.) 



Lastly, there is the logically unobservable ; namely, those things 

 which we cannot claim to have observed without breaking the laws 

 of reason. I doubt if this class is actual, since logic and observation 

 are essentially independent, but, as we shall see, it must be included 

 because a great deal has been written about it. An example might 

 be the observation of an object both larger and smaller than a 

 given object; but I give this example with some hesitation because 

 geometers have an uncanny knack of inventing spaces in which 

 such relations might not be incompatible. Be that as it may, how- 

 ever, if we grant a certain minimum of common agreement — such as 

 the acceptance of Euclidean geometry in the present instance — 

 logical unobservability becomes an intelligible notion, and we will 

 accept it as a candidate for inclusion in our principle. 



Now this classification may be simplified; for, whatever may be 

 the ultimate truth of the matter, it is not necessary for our purpose 

 to put the humanly and the physically unobservable in separate 

 classes. I will, therefore, group them together and call them jointly 

 the physically unobservable. The justification for this is that we 

 cannot tell, in any given case, with which class we are dealing. If, 

 for example, a certain substance appears tasteless to everyone (i. e., 

 its taste is unobservable), it is impossible to say whether that is 

 because it has a taste which our senses are not keen enough to detect, 

 or because it has no taste to be detected. There may be a distinction 

 between the two cases, but if so it is beyond our apprehension. But 

 now our principle is essentially one which, if valid, must be used; 

 it is not a creed which we are merely called upon to state and may 

 then ignore. The humanly and the physically unobservable, then, 

 become one class so far as our problem is concerned, for if in practice 

 we reject one, we automatically reject the other also. 



We have, then, three classes of unobservables, and I think the 

 distinction between them may be expressed most simply in the fol- 

 lowing way. Let us suppose that we have discovered all the means 

 of observation that exist in the universe, and know all their prop- 

 erties completely. We might then be able to imagine other means 

 of observation which do not exist. Anything which would be ob- 

 servable by such imaginary means, but not by the existing means, 

 would be physically unobservable. Anything which would be 



