WORLDS OF PHYSICS AND OF SENSE 105 



took the form of atomism. The underlying motive in 

 atomism was not, I think, any empirical success in inter- 

 preting phenomena, but rather an instinctive belief that 

 beneath all the changes of the sensible world there must 

 be something permanent and unchanging. This belief 

 was, no doubt, fostered and nourished by its practical 

 successes, culminating in the conservation of mass ; but 

 it was not produced by these successes. On the con- 

 trary, they were produced by it. Philosophical writers 

 on physics sometimes speak as though the conservation 

 of something or other were essential to the possibility 

 of science, but this, I believe, is an entirely erroneous 

 opinion. If the a priori belief in permanence had not 

 existed, the same laws which are now formulated in terms 

 of this belief might just as well have been formulated 

 without it. Why should we suppose that, when ice 

 melts, the water which replaces it is the same thing in a 

 new form ? Merely because this supposition enables us 

 to state the phenomena in a way which is consonant with 

 our prejudices. What we really know is that, under 

 certain conditions of temperature, the appearance we call 

 ice is replaced by the appearance we call water. We can 

 give laws according to which the one appearance will be 

 succeeded by the other, but there is no reason except 

 prejudice for regarding both as appearances of the same 

 substance. 



One task, if what has just been said is correct, which 

 confronts us in trying to connect the world of sense with 

 the world of physics, is the task of reconstructing the 

 conception of matter without the a priori beliefs which 

 historically gave rise to it. In spite of the revolutionary 

 results of modern physics, the empirical successes of the 

 conception of matter show that there must be some legiti- 

 mate conception which fulfils roughly the same functions. 



