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Let us, furthermore, bear in mind the great practical importance of distinguishing perceptions from ideas, however intensely recalled. Our whole relation to the external world, to reality, depends on our ability to do so. We have put forward the fiction that we did not always possess this ability and that at the beginning of our mental life we did in fact hallucinate the satisfying object when we felt the need for it. But in such a situation satisfaction did not occur, and this failure must very soon have moved us to create some contrivance with the help of which it was possible to distinguish such wishful perceptions from a real fulfilment and to avoid them for the future. In other words, we gave up hallucinatory satisfaction of our wishes at a very early period and set up a kind of ‘reality-testing’. The question now arises in what this reality-testing consisted, and how the hallucinatory wishful psychosis of dreams and amentia and similar conditions succeeds in abolishing it and in re-establishing the old mode of satisfaction.

The answer can be given if we now proceed to define more precisely the third of our psychical systems, the system Cs., which hitherto we have not sharply distinguished from the Pcs. In The Interpretation of Dreams we were already led to a decision to regard conscious perception as the function of a special system, to which we ascribed certain curious properties, and to which we shall now have good grounds for attributing other characteristics as well. We may regard this system, which is there called the Pcpt., as coinciding with the system Cs., on whose activity becoming conscious usually depends. Nevertheless, even so, the fact of a thing’s becoming conscious still does not wholly coincide with its belonging to a system, for we have learnt that it is possible to be aware of sensory mnemic images to which we cannot possibly allow a psychical location in the systems Cs. or Pcpt.

We must, however, put off discussing this difficulty till we can focus our interest upon the system Cs. itself. In the present connection we may be allowed to assume that hallucination consists in a cathexis of the system Cs. (Pcpt.), which, however, is not effected - as normally - from without, but from within, and that a necessary condition for the occurrence of hallucination is that regression shall be carried far enough to reach this system itself and in so doing be able to pass over reality-testing.¹

In an earlier passage² we ascribed to the still helpless organism a capacity for making a first orientation in the world by means of its perceptions, distinguishing ‘external’ and ‘internal’ according to their relation to its muscular action. A perception which is made to disappear by an action is recognized as external, as reality; where such an action makes no difference, the perception originates within the subject’s own body - it is not real. It is of value to the individual to possess a means such as this of recognizing reality, which at the same time helps him to deal with it, and he would be glad to be equipped with a similar power against the often merciless claims of his instincts. That is why he takes such pains to transpose outwards what becomes troublesome to him from within - that is, to project it.

¹ I may add by way of supplement that any attempt to explain hallucination would have to start out from negative rather than positive hallucination.

² ‘Instincts and their Vicissitudes’.