t glance
strikes the perception content created by the dream-work, while the next
strikes the one produced from without.

But of greater theoretical interest are those dreams which are capable
of waking us in the midst of sleep. We must bear in mind the expediency
elsewhere universally demonstrated, and ask ourselves why the dream or
the unconscious wish has the power to disturb sleep, _i.e._ the
fulfillment of the foreconscious wish. This is probably due to certain
relations of energy into which we have no insight. If we possessed such
insight we should probably find that the freedom given to the dream and
the expenditure of a certain amount of detached attention represent for
the dream an economy in energy, keeping in view the fact that the
unconscious must be held in check at night just as during the day. We
know from experience that the dream, even if it interrupts sleep,
repeatedly during the same night, still remains compatible with sleep.
We wake up for an instant, and immediately resume our sleep. It is like
driving off a fly during sleep, we awake _ad hoc_, and when we resume
our sleep we have removed the disturbance. As demonstrated by familiar
examples from the sleep of wet nurses, &c., the fulfillment of the wish
to sleep is quite compatible with the retention of a certain amount of
attention in a given direction.

But we must here take cognizance of an objection that is based on a
better knowledge of the unconscious processes. Although we have
ourselves described the unconscious wishes as always active, we have,
nevertheless, asserted that they are not sufficiently strong during the
day to make themselves perceptible. But when we sleep, and the
unconscious wish has shown its power to form a dream, and with it to
awaken the foreconscious, why, then, does this power become exhausted
after the dream has been taken cognizance of? Would it not seem more
probable that the dream should continually renew itself, like the
troublesome fly which, when driven away, takes pleasure in returning
again and again? What justifies our assertion that the dream removes the
disturbance of sleep?

That the unconscious wishes always remain active is quite true. They
represent paths which are passable whenever a sum of excitement makes
use of them. Moreover, a remarkable peculiarity of the unconscious
processes is the fact that they remain indestructible. Nothing can be
brought to an end in the unconscious; nothing can cease or