must also of course be liable to disturbance by morbid processes. From pathology we have come to know a large number of states in which the boundary lines between ego and outer world become uncertain, or in which they are actually incorrectly perceived—cases in which parts of a man’s own body, even component parts of his own mind, perceptions, thoughts, feelings, appear to him alien and not belonging to himself; other cases in which a man ascribes to the external world things that clearly originate in himself, and that ought to be acknowledged by him. So the ego’s cognizance of itself is subject to disturbance, and the boundaries between it and the outer world are not immovable. Further reflection shows that the adult’s sense of his own ego cannot have been the same from the beginning. It must have undergone a development, which naturally cannot be demonstrated, but which admits of reconstruction with a fair degree of probability.[2] When the infant at the breast receives stimuli, he cannot as yet distinguish whether they come from his ego or from the outer world. He learns it gradually as the result of various exigencies. It must make the strongest impression on him that many sources of excitation, which later on he will recognize as his own bodily organs, can provide him at any time with sensations, whereas others become temporarily out of his reach—amongst these what he wants most of all, his mother’s breast—and reappear only as a result of his cries for help. Thus an ‘object’ first presents itself to the ego as something existing ‘outside’, which is only induced to appear by a particular act. A further stimulus to the growth and formation of the ego, so that it becomes something more than a bundle of sensations, _i.e._ recognizes an ‘outside’, the external world, is afforded by the frequent, unavoidable and manifold pains and unpleasant sensations which the pleasure-principle, still in unrestricted domination, bids it abolish or avoid. The tendency arises to dissociate from the ego everything which can give rise to pain, to cast it out and create a pure pleasure-ego, in contrast to a threatening ‘outside’, not-self. The limits of this primitive pleasure-ego cannot escape readjustment through experience. Much that the individual wants to retain because it is pleasure-giving is nevertheless part not of the ego but of an object; and much that he wishes to eject because it torments him yet proves to be inseparable from the ego, arising from an inner source. He learns a method by which, through deliberate use of the sensory organs and suitable muscular movements, he can distinguish between internal and external—what is part of the ego and what originates in the outer world—and thus he makes the first step towards the introduction of the reality-principle which is to control his development further. This capacity for distinguishing which he learns of course serves a practical purpose, that of enabling him to defend himself against