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.