CONCLUSION 341 



to obscure the identity of the constituent strands. 

 ^Esthetic and religious emotions, for example, may 

 show no obvious traces of their dependence on these 

 instinctive elements without the aid of close scrutiny. 

 The history of the development of the nervous system 

 itself from unspecialized cells supplies a material anal- 

 ogy to the idea that superior mental states may arise 

 from humble origins of unpredictable potentialities. 

 The psychical life of the normal individual is based 

 on three kinds of experiences. Two of these have 

 their origin in stimuli from the outer world, either 

 from unorganized nature or from living beings. The 

 stimuli derived from association with human beings 

 are responsible for some of the most important forma- 

 tive processes of the human nervous system, and one 

 may go so far as to say that the continual and multi- 

 plex interaction of nervous protoplasm through 

 the agency of what we call mind is the prime 

 environmental agency in the evolution of ideas and 

 concepts. But the stimuli from the outer world are 

 recorded physically in the neural structures, and 

 these records are susceptible of revival from within 

 the nervous system itself; that is, without the aid of 

 new external stimuli. This third type of human 

 experience, the revival of sensory impressions and of 

 the mental states accompanying them, is the physi- 

 ological basis of thought, imagination, reflection 

 or in other words, of psychical growth. Such 

 revivals of sensory impressions are of course not 

 limited to this inner origin, but are very generally 

 initiated by stimuli through the sense organs. 



