THE VITAL IMPETUS 159 



of mechanisms) out of many. A motor habit, or path, 

 is then established and will persist. 



Such a conception is clear and reasonable in principle, 

 and all work on nervous physiology tends to show that 

 it is a good working hypothesis. We cannot read 

 modern books without feeling that immense advances 

 will be made by its aid. But the complexity of the 

 brain of the higher vertebrate is so incredibly great, 

 and the difficulties of imagining the nature of the 

 necessary physico-chemical reactions in the synapses, 

 and elsewhere, are so immense that experimental 

 verification may be impossible. And all that we have 

 said applies to a single elemental stimulus, yet in any 

 common action the stimulus is a synthesis of almost 

 innumerable simple ones, while the response is also a 

 synthesis. The optical image of almost any object 

 contains a very great number of tints and colours 

 differing almost imperceptibly : there must at least 

 be as many simple stimuli as there are rod or cone 

 elements in the part of the retina covered by the 

 image. The motor responses consist of a multitude of 

 delicately adjusted and co-ordinated muscular con- 

 tractions and relaxations. If we are to accept a 

 mechanistic hypothesis of action, of this kind, and 

 which includes only such processes as are suggested 

 above, it is not enough that a logical description, con- 

 sistent in itself, and consistent with physico-chemical 

 knowledge, should be formulated. The mere state- 

 ment of such an hypothesis does not carry us far. If 

 it is, in essence, mechanistic, it must be capable of 

 experimental verification in detail. 



Even if it were verified experimentally it would 

 still leave untouched the problem of consciousness. 

 All that we have considered are series of physico- 

 chemical energy-transformations. How, then, does 



