322 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



reluctance of the human mind to leave the empirical realm. 

 Similarly, the generalization of the scientific concept of 

 motion to include velocities beyond the limits of observa- 

 tion may be described as an operation of abstraction, or, if 

 one prefers, of serial extension. An instantaneous velocity 

 could be defined either as the limit of velocities of con- 

 tinually decreasing durations, or as the series itself of such 

 velocities, as in the case of Whitehead's definition of " point." 

 The distinction between the concept of motion as it func- 

 tions in the classical physics, and the notion as it plays a 

 part in the recent relativity theory is based upon a recog- 

 nition of the elusiveness of absolute velocity through space. 

 Newton himself, apparently, realized the hypothetical char- 

 acter of absolute motion, since he pointed out that the laws 

 of mechanics have the same form in all reference systems 

 moving with respect to each other with constant velocity. 

 This was equivalent to admitting that by no mechanical 

 experiment could absolute motion of this kind be deter- 

 mined. "No mechanical experiment that can be performed, 

 for example, on a train moving at constant speed can 

 divulge to us that the train is really moving, for the results 

 of all such experiments relative to the train are precisely the 

 same as those which would have been obtained by the 

 performance of similar experiments at the station or on the 

 roadside." 1 It was supposed, however, that such absolute 

 velocity, i.e., velocity relative to some absolutely fixed point 

 such as the ether, could be detected by other experiments, 

 e.g., optical ones. The Michelson-Morley experiment was 

 designed to verify precisely such a supposition. But its 

 negative results, and the negative results of similar experi- 

 ments performed upon electro-magnetic phenomena in 

 general, have suggested that perhaps absolute velocity 

 cannot be detected by any means; and the suspicion im- 

 mediately arises that absolute velocity may be meaningless. 

 At least if absolute motion cannot be established by any 

 instrumental techniques, and if it cannot be logically derived 



1 R. B. Lindsay and H. Margenau, Foundations of Physics, p. 331. 



