PHYSICS, BIOLOGY, AND PSYCHOLOGY. 155 



sation carries interpretation with it, as Kant showed. 

 The " objective " world is nothing but the world as 

 interpreted in knowledge, and the physical or biological 

 worlds are only abstractions from this objective world. 

 Not only when we are observing psychological pheno- 

 mena in other persons, but when we are studying 

 natural phenomena of all kinds, is our world a psycho- 

 logical or spiritual world. Perhaps we realise this best 

 when the progress of experimental science leads to a 

 reconsideration of fundamental physical interpretations 

 which, like those of mass, energy, or unchangeable 

 atoms, have been .employed without question for long 

 periods. We have to go back to what was in the minds 

 of those who established these interpretations. 



I will now try to summarise the argument of this 

 paper. When we make use of physical categories we 

 are employing simplified maxims or principles which, 

 on account of their simplicity, are very convenient for 

 purposes of prediction, but which can only be used over 

 a limited extent of our experience without gross error. 

 When we attempt to apply them to biological or psycho- 

 logical phenomena, the error becomes apparent ; we 

 cannot express biological or psychological experience 

 in terms of physical conceptions. In other words, we 

 cannot reduce biological and psychological to physical 

 categories. 



Similarly, in biology we are also employing relatively 

 simplified maxims which enable us to predict another 

 large class of phenomena, but cannot be applied to what 

 we distinguish as psychological phenomena without 

 gross error. Hence we cannot reduce psychological to 

 biological categories. 



