From this follows the curious consequence that the mind is endowed with an incomplete existence; it is like form, which can only be realised by its application to matter of some kind. One may fancy a sensation continuing to exist, to live and to provoke movements, even after ceasing to be perceived. Those who are not uncompromising idealists readily admit this independence of the objects with regard to our consciousness, but the converse is not true. It is impossible to understand a consciousness existing without an object, a perception without a sensation to be perceived, an attention without a point of application, an empty wish which should have nothing to wish for; in a word, a spiritual activity acting without matter on which to act, or more briefly still--mind without matter. Mind and matter are correlative terms; and, on this point, I firmly believe that Aristotle was much closer to the truth than many modern thinkers. I have convinced myself that the definition of mind at which we have just arrived is, in its exactness and soberness, the only one which permits psychology to be distinguished from the sciences nearest to it. You know that it has been discovered in our days that there exists a great difficulty in effecting this delimitation. The definitions of psychology hitherto proposed nearly all have the defect of not agreeing with the one thing defined. Time fails us to review them all, but I shall point out one at least, because our discussion on this particular formula will serve as a preparation for taking in hand the last question that remains to be examined--the relation of the mind to the body. According to the definition I am aiming at, psychology would be the science of internal facts, while the other sciences deal with the external. Psychology, it has also been said, has as its instrument introspection, while the natural sciences work with the eye, the touch, the ear--that is to say, with the senses of extrospection. To this distinction, I reply that in all sciences there exist but two things: sensations and the consciousness which accompanies them. A sensation may belong to the inner or the outer world through accidental reasons, without any change in its nature; the sensation of the outer world is the social sensation which we share with our fellows. If the excitant which provokes it is included in our nervous system, it is the sensation which becomes individual, hidden to all except ourselves, and constituting a microcosm by the side of a macrocosm. What importance can this have, since all the difference depends on the position occupied by the excitant?