Finally, it is to be observed that, while the centres thus respond with diminished energy to peripheral stimuli, external and internal, they undergo a direct, or “automatic,” mode of excitation, being roused into activity independently of an incoming nervous impulse.  This automatic stimulation has been plausibly referred to the action of the products of decomposition accumulating in the cerebral blood-vessels.[77] It is possible that there is something in the nature of this stimulation to account for the force and vividness of its conscious results, that is to say, of dreams.

The Dream State.

Let us now turn to the psychic side of these conditions, that is to say, to the general character of the mental states known as dreams.  It is plain that the closing of the avenues of the external senses, which is the accompaniment of sleep, will make an immense difference in the mental events of the time.  Instead of drawing its knowledge from without, noting its bearings in relation to the environment, the mind will now be given over to the play of internal imagination.  The activity of fancy will, it is plain, be unrestricted by collision with external fact.  The internal mental life will expand in free picturesque movement.

To say that in sleep the mind is given over to its own imaginings, is to say that the mental life in these circumstances will reflect the individual temperament and mental history.  For the play of imagination at any time follows the lines of our past experience more closely than would at first appear, and being coloured with emotion, will reflect the predominant emotional impulses of the individual mind.  Hence the saying of Heraclitus, that, while in waking we all have a common world, in sleep we have each a world of our own.

This play of imagination in sleep is furthered by the peculiar attitude of attention.  When asleep the voluntary guidance of attention ceases; its direction is to a large extent determined by the contents of the mind at the moment.  Instead of holding the images and ideas, and combining them according to some rational end, the attention relaxes its energies and succumbs to the force of imagination.  And thus, in sleep, just as in the condition of reverie or day-dreaming, there is an abandonment of the fancy to its own wild ways.

It follows that the dream-state will not appear to the mind as one of fancy, but as one of actual perception, and of contact with present reality.  Dreams are clearly illusory, and, unlike the illusions of waking life, are complete and persistent.[78] And the reason of this ought now to be clear.  First of all, the mind during sleep wants what M. Taine calls the corrective of a present sensation.  When awake under ordinary circumstances, any momentary illusion is at once set right by a new act of orientation.  The superior vividness of the external impression cannot leave us in any doubt, when calm and self-possessed,