These co-excitations of parts of the brain functionally different, on occasion of excitation of a part of the brain that has previously often been excited together with those, form the physiological foundation of the psychical phenomenon of the formation of concepts in general, and so of the formation of the "I"-concept. For the special ideas of all departments of sense have in all beings possessed of all the senses--or of four senses, or of three--the common quality of coming into existence only under conditions of time, space, and causality. This common property presupposes similar processes in every separate sense-center of the highest rank. Excitations of one of these centers easily effect similar co-excitations of centers that have often been excited together with them through objective impressions, and it is this similar co-excitement extending itself over the cerebral centers of all the nerves of sense that evokes the composite idea of the "I." According to this view, therefore, the "I" can not exist as a unit, as undivided, as uninterrupted; it exists only when the separate departments of sense are active with their _egos_, out of which the "I" is abstracted; e. g., it disappears in dreamless sleep. In the waking condition it has continued existence only where the centro-sensory excitations are most strongly in force; i. e., where the attention is on the strain. Still less, however, is the "I" an aggregate. For this presupposes the exchangeability of the component parts. The seeing _ego_, however, can just as little have its place made good by a substitute as can the hearing one, the tasting one, etc. The sum-total of the separate leaves, blossoms, stalks, roots, of the plant does not, by a great deal, constitute the plant. The parts must be joined together in a special manner. So, likewise, it is not enough to add together the characteristics common to the separate sense-representations in order to obtain from these the regulating and controlling "I." Rather there results from the increasing number and manifoldness of the sense-impressions a continually increasing growth of the gray substance of the child's cerebrum, a rapid increase of the intercentral connecting fibers, and through this a readier co-excitement--association, so called--which unites feeling with willing and thinking in the child. This union is the "I," the sentient and emotive, the desiring and willing, the perceiving and thinking "I." CHAPTER XX. SUMMARY OF RESULTS. Of all the facts that have been established by me through the observation of the child in the first years of his life, the _formation of concepts without language_ is most opposed to the traditional doctrines, and it is just this on which I lay the greatest stress.