We have not yet exhausted all the concepts whereby we may get to unconsciousness. Here is another, the last I shall quote, without, however, claiming that it is the last which exists. We might call it the physiological concept, for it is the one which the physiologists employ for choice. It is based upon the observation of the phenomena which are produced in the nervous system during our acts of consciousness; these phenomena precede consciousness as a rule, and condition it. According to a convenient figure which has been long in use, the relations of the physiological phenomenon to the consciousness are represented as follows: the physiological phenomenon consists in an excitement which, at one time, follows a direct and short route from the door by which it enters the nervous system to the door by which it makes its exit. In this case, it works like a simple mechanical phenomenon; but sometimes it makes a longer journey, and takes a circuitous road by which it passes into the higher nerve centres, and it is at the moment when it takes this circuitous road that the phenomenon of consciousness is produced. The use of this figure does not prejudge any important question. Going further, many contemporary authors do not content themselves with the proposition that the consciousness is conditioned by the nervous phenomenon, but suggest also that it is continually accompanied by it. Every psychical fact of perception, of emotion, or of idea should have, it is supposed, a physiological basis. It would therefore be, taken in its entirety, psycho-physiological. This is called the parallelist theory. We cannot discuss this here, as we shall meet with it again in the third part of this book. It has the advantage of leading to a very simple definition of unconsciousness. The unconscious is that which is purely physiological. We represent to ourselves the mechanical part of the total phenomenon continuing to produce itself, in the absence of the consciousness, as if this last continued to follow and illuminate it. Such are the principal conceptions that may be formed of the unconscious. They are probably not the only ones, and our list is not exhaustive. After having indicated what the unconscious is, we will terminate by pointing out what it is not and what it cannot be.