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In what way, however, can a case arise in which the intention to sleep meets with an interruption? The interruption may proceed from an internal excitation or from an external stimulus. Let us first consider the more obscure and more interesting case of interruption from within. Observation shows that dreams are instigated by residues from the previous day - thought-cathexes which have not submitted to the general withdrawal of cathexes, but have retained in spite of it a certain amount of libidinal or other interest. Thus the narcissism of sleep has from the outset had to admit an exception at this point, and it is here that the formation of dreams takes its start. In analysis we make the acquaintance of these ‘day’s residues’ in the shape of latent dream-thoughts; and, both by reason of their nature and of the whole situation, we must regard them as preconscious ideas, as belonging to the system Pcs.

We cannot proceed any further in explaining the formation of dreams till we have overcome certain difficulties. The narcissism of the state of sleep implies a withdrawal of cathexis from all ideas of objects, from both the unconscious and the preconscious portions of those ideas. If, then, certain day’s residues have retained their cathexis, we hesitate to suppose that they have acquired at night so much energy as to compel notice on the part of consciousness; we should be more inclined to suppose that the cathexis they have retained is far weaker than that which they possessed during the day. Here analysis saves us further speculation, for it shows that these day’s residues must receive a reinforcement which has its source in unconscious instinctual impulses if they are to figure as constructors of dreams. This hypothesis presents no immediate difficulties, for we have every reason to suppose that in sleep the censorship between the Pcs. and the Ucs. is greatly reduced, so that communication between the two systems is made easier.