CLASSIFICATION OF KNOWLEDGE 9 



represent our model of science as a sequence of sub- 

 jects, with physical science at one end, biology in 

 the middle, and psychology at the other end a 

 sequence like that of the colours of the rainbow, 

 where red represents physics, green biology, and blue 

 psychology, physics appearing out of the invisible 

 as a faint, purplish glow, and psychology fading away 

 into a haze of violet, not very different from the 

 purple basis of physics. 



Purple Red. Yellow Green Blue-Green Blue 



FIG. 1. 



And, indeed, we are led back by psychology to 

 approach logic, physics, and the other sciences we 

 have traced, from a new standpoint. Psychology 

 gives us insight into the theory of knowledge, and 

 leads us critically to examine how it is that we know 

 anything, either by logic or by the senses, or by that 

 fertile union of the two which breeds science. Thus, 

 by the road of psychology, we return once more to our 

 starting-point, ready to study fundamental, logical, 

 mechanical, and physical ideas from a critical point 

 of view. 



We shall do well, then, to bend our spectrum round 

 so that the violet theory of knowledge, which lies 

 beyond the blue of psychology, shall be brought near 

 to the purple of logic, where fundamental scientific 



