INTRODUCTION vii 



of the conservation of matter and energy. There are 

 existences which may or may not persist. Visions 

 and phantasms and dreams are existences while they 

 last. They are true for the mind in which they occur. 

 But they seem to arise out of nothing, and to dis- 

 appear into nothing, and physical Science cannot 

 investigate them. They are existences which are not 

 conserved. On the other hand those images which we 

 call moving matter and transforming energy can be 

 investigated by the methods of physics. Molecules 

 change, but something in them, the atoms, remain 

 constant. Energy becomes transformed, and it may 

 even seem to cease to exist, but if it disappears, then 

 something is changed so that the lost energy can be 

 traced in the nature of the change. Matter and 

 energy are conserved and therefore they are the only 

 Realities. But the test is obviously one that has an 

 a priori basis, and we may doubt whether it is a test 

 of Reality. 



Thus Physics constructed a dynamical Universe, 

 that is, one which consisted of atoms which attracted 

 or repelled each other with forces which were functions 

 of the distances between them. Even now this con- 

 ception of a dynamical, Newtonian Universe is a use- 

 ful one, though we recognise that it is only symbolism. 

 But it was not a conception with which Physics could 

 long remain content. How could atoms separated 

 from each other by empty space act on each other, 

 that is, how could a thing act where it was not ? 

 There must be something between the atoms. The 

 Universe could not be a discontinuous one, and so 

 Physics invented an Universe that was full. It was 

 an immaterial, homogeneous, imponderable, con- 

 tinuous Universe. That which existed behind the 

 appearances of atoms and molecules and energy was 



