WORLDS OF PHYSICS AND OF SENSE 109 



pose this that we assume intermediate unobserved states. 

 Continuity is also not sufficient, since we can, for example, 

 pass by sensibly continuous gradations from any one drop 

 of the sea to any other drop. The utmost we can say is 

 that discontinuity during uninterrupted observation is as 

 a rule a mark of difference between things, though even 

 this cannot be said in such cases as sudden explosions. 



The assumption of continuity is, however, successfully 

 made in physics. This proves something, though not 

 anything of very obvious utility to our present problem : 

 it proves that nothing in the known world is inconsistent 

 with the hypothesis that all changes are really continuous, 

 though from too great rapidity or from our lack of 

 observation they may not always appear continuous. In 

 this hypothetical sense, continuity may be allowed to be a 

 necessary condition if two appearances are to be classed as 

 appearances of the same thing. But it is not a sufficient 

 condition, as appears from the instance of the drops in 

 the sea. Thus something more must be sought before 

 we can give even the roughest definition of a " thing." 



What is wanted further seems to be something in the 

 nature of fulfilment of causal laws. This statement, as it 

 stands, is very vague, but we will endeavour to give it 

 precision. When I speak of " causal laws," I mean any 

 laws which connect events at different times, or even, as 

 a limiting case, events at the same time provided the 

 connection is not logically demonstrable. In this very 

 general sense, the laws of dynamics are causal laws, and 

 so are the laws correlating the simultaneous appearances 

 of one " thing ' to different senses. The question is : 

 How do such laws help in the definition of a " thing " ? 



To answer this question, we must consider what it is 

 that is proved by the empirical success of physics. 

 What is proved is that its hypotheses, though unverifiable 



