entral
activity, however diminished during sleep, always retains a minimum
degree of intensity. At least, one would be disposed to argue in this
way from the analogy of the condition of the other functions of the
organism during sleep. Possibly this modicum of positive evidence may
more than outweigh any slight presumption against the doctrine of
unbroken mental activity drawn from the negative circumstance that we
remember so little of our dream-life.[75]

Such being the state of physiological knowledge respecting the
immediate conditions of sleep, we cannot look for any certain
information on the nature of that residual mode of cerebral activity
which manifests itself subjectively in dreams. It is evident, indeed,
that this question can only be fully answered when the condition of the
brain as a whole during sleep is understood. Meanwhile we must be
content with vague hypotheses.

It may be said, for one thing, that during sleep the nervous substance
as a whole is less irritable than during waking hours. That is to say, a
greater amount of stimulus is needed to produce any conscious
result.[76] This appears plainly enough in the case of the peripheral
sense-organs. Although these are not, as it is often supposed, wholly
inactive during sleep, they certainly require a more potent external
stimulus to rouse them to action. And what applies to the peripheral
regions applies to the centres. In truth, it is clearly impossible to
distinguish between the diminished irritability of the peripheral and
that of the central structures.

At first sight it seems contradictory to the above to say that stimuli
which have little effect on the centres of consciousness during waking
life produce an appreciable result in sleep. Nevertheless, it will be
found that this is the case. Thus organic processes which scarcely make
themselves known to the mind in a waking state, may be shown to be the
originators of many of our dreams. This fact can only be explained on
the physical side by saying that the special cerebral activities
engaged in an act of attention are greatly liberated during sleep by the
comparative quiescence of the external senses. These activities, by
co-operating with the faint results of the stimuli coming from the
internal organs, serve very materially to increase their effect.

Finally, it is to be observed that, while the centres thus respond with
diminished energy to peripheral stimuli, external and internal, they