In sleep, however, these groups are not usually so firmly held together by the cords along which we can move in our waking moments from one to the other. They are, as it were, loosened from their moorings, and on the sea of sleeping consciousness they drift apart or jostle together in new and what seem to be random associations. This is that process of dissociation which we find so marked in dreaming, and in all those psychic phenomena--hallucinations, hysteria, multiple personality, insanity--which are allied to dreaming. A simple illustration of the clash and confusion of two opposing systems of memories in dreams, when due apperceptive control is lacking, is supplied by a common and well-recognised type of dream, the dream of returning to the school of youth.[178] Many people are occasionally liable to this dream, which is often vivid and disturbing. We may have left the schoolroom thirty years or more ago, and never seen it since; it may have vanished from our waking thoughts. Yet from time to time we find ourselves there in our dreams, and called upon to take our old place, always with a sense of conflict, a vague discomfort, a feeling of something incongruous and humiliating, for we realise that we are now too old. Here is a dream in illustration: I find myself back at my old school, but my old schoolmaster is not there; he is away ill, as I am told by his substitute, whose face somehow seems familiar, though I cannot recall where I have seen it. I do not know any of the boys; I am returning after an absence of some months. I realise that I am to take my old place again, and yet I feel a profound repulsion to do so, a sense that it is somehow incongruous. This latter feeling seems to prevail, for I finally assume the part of a visitor, and remark, insincerely, to the master that it is pleasant to see the old place again. In such a case as this it seems that a picture from an ancient system of memories floats across the field of sleeping consciousness, and the dreamer is naturally drawn into that system and begins to adapt himself to its demands. But, as he does so, the influence of other later and incompatible systems of memories begins unconsciously to affect the dreamer.[179] The cords of connection, however, which when awake would enable him to adjust critically the opposing systems, are not acting; apperception is defective. Yet the opposing systems are there, outside the immediate field of consciousness, and jostling the ancient system which has come into the central focus. Finally this jostling of the ancient system by more recent systems causes a harmonising modification in consciousness. The dreamer ceases to be a boy in his old school, and assumes the part of a visitor.