The Domain of Physical Science 193 



renounce the idea that these are privileged circumstances ; the 

 purpose of conceiving an external world is to obtain a conception 

 which could be shared by beings in any other physical circumstances 

 whatever. 



The external world is accordingly a synthesis of appearances 

 from all possible points of view. In the main, modern science 

 accepts this principle and arrives at its adopted conception of our 

 environment by following it. The man and the microbe afford 

 only one example of the possible variety of points of view. Recently 

 physicists have been much occupied in comparing the points of 

 view of observers travelling with different motions, e.g.^ attached 

 to different stars. The result has been entirely to revolutionise the 

 conception of space and time in the external world. The detailed 

 frame of space and time in which we are accustomed to locate 

 the events happening around us belongs not to the external world 

 but to a particular presentation of it — namely, to those observers 

 who are travelling with the same velocity as the earth. A being 

 on a star with different velocity would, if he followed our methods 

 and assumptions, obtain a different reckoning of space and time, 

 and his location of external events would be a distorted version of 

 our own. In the external world, which is a synthesis of all points 

 of view, we cannot give preference to one version rather than the 

 other ; space and time, in the form in which we commonly repre- 

 sent them, cannot belong to the external world. The work of 

 Einstein and Minkowski has shown how the synthesis is to be 

 made ; it leads to the conception of a four-dimensional space- 

 time {i.e., a fourfold order of events) in which there is no straight- 

 cut separation into space and time, although there is a definite 

 structural arrangement on a rather simple plan which is the genesis 

 of the separation by the various possible observers. Following up 

 this success, Einstein began to synthesise the points of view of 

 observers differing not merely in uniform velocity but in accelera- 

 tion of velocity, e.g., a man on terra firma and a man falling from a 

 precipice. Strange as it may seem, this bold extension of the 

 principle of synthesis — this refusal to reject any natural point of 

 view as a " wrong " one — led to a striking success. It was found 

 that gravitation, previously a deus ex machina in physical science, 

 became incorporated in the results of the synthesis ; that is to say, 

 the conception of the external world as modified so as to include 

 the additional points of view, predicted without further hypothesis 



