The second question is that of knowing what is the rôle, the utility, and the efficacity of the psychical phenomenon. Once formed, this phenomenon evolves in a certain direction and assumes to us who have consciousness of it a very great importance. What is its action on the material phenomena of the brain which surround it? Does it develop according to laws of its own, which have no relation to the laws of brain action? Does it exercise any action on these intra-cerebral functions? Does it exercise any action on the centrifugal currents which go to the motor nerves? Is it capable of exciting a movement? or is it deprived of all power of creating effect?
We will briefly examine the principal solutions which the imagination of mankind has found for these very difficult problems. Some of the best known of these solutions bear the names of spiritualism, materialism, parallelism, and monism. We will speak of these and of some others also.
Before beginning our critical statement, let us recall some of the results of our previous analyses which here intrude themselves, to use the ambitious language of Kant, as the prolegomena to every future solution which claims the title of science. In fact, we are now no longer at the outset of our investigation. We have had to acknowledge the exactness of certain facts, and we are bound to admit their consequences.
