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, whether our mental images answer to present realities or not.

On the other hand, when asleep, this reference to a fixed objective standard is clearly impossible.

Secondly, we may fairly argue that the mental images of sleep approximate in character to external impressions.