THE study of the cerebral associative mechanisms on which the processes of perception and the forms of thought depend does not exhaust the problem of mental activity.
The dynamic standpoint of evolutionary biology, which dominates contemporary psychology, demands that we should occupy ourselves with the actual operation of these mechanisms. When we consider mental functioning in action, even assuming that we know all the groups of neurones implicated in a given process, we still have to determine what causes this process, and on what energy it draws. There is a quantitative aspect in mental action seen as a whole, with which the remarkable work of Pierre Janet is connected. And this quantitative aspect involves complicated distinctions.!