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We have now to consider the further vicissitudes undergone by this wishful
impulse, which in its essence represents an unconscious instinctual demand and
which has been formed in the Pcs. as a dream-wish (a wish-fulfilling phantasy). Reflection tells us that this
wishful impulse may be dealt with along three different paths. It may follow the
path that would be normal in waking life, by pressing from the Pcs. to consciousness; or it may bypass the Cs. and find direct motor discharge; or it may take the unexpected path which
observation enables us in fact to trace. In the first case, it would become a delusion having as content the fulfilment of the wish; but in the state of sleep this
never happens. With our scanty knowledge of the metapsychological conditions of
mental processes, we may perhaps take this fact as a hint that a complete
emptying of a system renders it little susceptible to instigation. The second case,
that of direct motor discharge, should be excluded by the same principle; for
access to motility normally lies yet another step beyond the censorship of
consciousness. But we do meet with exceptional instances in which this happens, in
the form of somnambulism. We do not know what conditions make this possible, or
why it does not happen more often. What actually happens in dream-formation is
a very remarkable and quite unforeseen turn of events. The process, begun in
the Pcs. and reinforced by the Ucs., pursues a backward course, through the Ucs. to perception, which is pressing upon consciousness. This regression is the third phase of dream-formation. For the sake of clarity, we will
repeat the two earlier ones: the reinforcement of the Pcs. by the Ucs., and the setting up of the dream-wish.
We call this kind of regression a topographical one, to distinguish it from the previously mentioned temporal or developmental regression. The two do not necessarily always coincide, but
they do so in the particular example before us. The reversal of the course of
the excitation from the Pcs. through the Ucs. to perception is at the same time a return to the early stage of
hallucinatory wish-fulfilment.