ep the
withdrawal of the mind from the external world is more complete and the
objectivity of the dream images is usually unquestioned, whereas in the
waking state the hallucination is usually recognized as such; we may,
however, be conscious that we are dreaming, and thus in a measure be
aware of the hallucinatory character of our percepts. The physiological
nature of sleep (q.v.; see also MUSCLE AND NERVE) and of dreaming is
obscure. As a rule the control over the voluntary muscles in dreams is
slight; the sleep-walker is the exception and not the rule, and the
motor activity represented in the dream is seldom realized in practice,
largely, no doubt, because we are ignorant, under these circumstances,
of the spatial relations of our bodies. Among the psychological problems
raised by dreams are the condition of attention, which is variously
regarded as altogether absent or as fixed, the extent of mental control,
and the relation of ideas and motor impulses. There is present in all
dreams a certain amount of dissociation of consciousness, or of
obstructed association, which may manifest itself in the preliminary
stage of drowsiness by such phenomena as the apparent transformation or
inversion of the words of a book. We may distinguish two types of
dreams, (a) representative or centrally initiated, (b) presentative or
due to the stimulation of the end organs of sense. In both cases, the
dream having once been initiated, we are concerned with a process of
reasoning, i.e. the combination of ideas suggested by resemblances or
other associative elements. The false reasoning of dreams is due in the
first place to the absence, to a large extent, of the memory elements on
which our ordinary reasoning depends, and, secondly, to the absence of
sensory elements.

_Objectivity of Dreams._--In waking life we distinguish ideas or mental
images from real objects by the fact that we are able under normal
circumstances to dismiss the former at will. In sleep, on the other
hand, we have, in the first place, no real objects with which to compare
the images, which therefore take on a character of reality comparable to
the hallucination of waking life; moreover, powers of visualization and
other faculties are enhanced in sleep, so that the strength of dream
images considerably exceeds those of the mental images of the ordinary
man; changes in powers of attention, volition and memory help to
increase the hallucinatory force of the dream. In