The Domain of Physical Science 203 



contrivances called brains. The physicist, I suppose, admits con- 

 sciousness to a place in the world because he cannot very well deny 

 it ; but it would seem that he has no use for it in his scheme. Like 

 Laplace, he has no need of any such hypothesis. The religious 

 mind is naturally jealous of the thought that there can be such a 

 domain of reality pursuing its course independently of all spiritual 

 things. It would welcome an admission from the physicist that his 

 material world is not self-sufficient and would dissolve if it were 

 not sustained by a spiritual reality, which, it is felt, must be deeper 

 than all material reality. But suppose the physicist does see the 

 error of his ways ; suppose he does find it necessary to include an 

 all-pervading spirit among his hypotheses, then even greater appre- 

 hension is felt. The physicist is at once suspected of a design to 

 reduce God to a system of differential equations, for it is difficult 

 to see what place in physics there can be for a hypothesis not 

 reducible to this form. We must see whether the recent theories 

 of the physical world help to steer a path between these difficulties. 

 Our discussion has already prepared us to admit that physics (or 

 exact science) can only take within its scope certain aspects of 

 the external world ; and that there remain other aspects which 

 have been excluded, not because they are of less importance, but 

 because they have not the specialised property of measurability. 

 The difficulty does not lie in recognising a wider spiritual reality 

 from which the physical world is a specialised selection. The 

 difficulty is to explain why the physical world, picked out from 

 a more comprehensive world by the criterion of measurability, 

 should be found to constitute a self-contained system ; it operates 

 with so little interference from the rest of reality that we often 

 forget that it is only a part. This problem of the self-sufficiency 

 of the physical world must now be considered. 



3. Physical Science as a Closed System 



The central point of Einstein's great theory is a new law of 

 gravitation approximating to but more accurate than Newton's 

 law, and we shall start by explaining exactly the formulation of 

 this law. Our explanation will be more thorough than usual, for 

 we shall make a point of defining each new term that it is necessary 

 to introduce. Probably the effect of giving so full an explanation 

 will be that the reader will not understand the new law any better 



