The first stage in mental development is the establishment of regular and uniform relations between soul and body: it is the differentiation of organs and the integration of function: the balance between sensation and movement, between the afferent and efferent processes of sensitivity. Given a potential soul, the problem is to make it actual in an individual body. It is the business of a physical psychology to describe in detail the steps by which the body we are attached to is made inward as our idea through the several organs and their nervous appurtenances: whereas a psychical physiology would conversely explain the corresponding processes for the expression of the emotions and for the objectification of the volitions. Thus soul inwardises (_erinnert_) or envelops body: which body “expresses” or develops soul. The actual soul is the unity of both, is the percipient individual. The solidarity or “communion” of body and soul is here the dominant fact: the soul sentient of changes in its peripheral organs, and transmitting emotion and volition into physical effect. It is on this psychical unity,—the unity which is the soul of the diversity of body—that all the subsequent developments of mind rest. Sensation is thus the _prius_—or basis—of all mental life: the organisation of soul in body and of body in soul. It is the process which historically has been prepared in the evolution of animal life from those undifferentiated forms where specialised organs are yet unknown, and which each individual has further to realise and complete for himself, by learning to see and hear, and use his limbs. At first, moreover, it begins from many separate centres and only through much collision and mutual compliance arrives at comparative uniformity and centralisation. The common basis of united sensibility supplied by the one organism has to be made real and effective, and it is so at first by sporadic and comparatively independent developments. If self-hood means reference to self of what is prima facie not self, and projection of self therein, there is in primitive sensibility only the germ or possibility of self-hood. In the early phases of psychic development the centre is fluctuating and ill-defined, and it takes time and trouble to co-ordinate or unify the various starting-points of sensibility(81).