Q: Is that which is termed “unconscious cerebration” during sleep a mechanical process of the physical brain, or is it a conscious operation of the Ego, the result of which only is impressed on the ordinary consciousness?
A: It is the latter; for is it possible to remember in our conscious state what took place while our brain worked unconsciously? This is apparently a contradiction in terms.
Q: How does it happen that persons who have never seen mountains in nature often see them distinctly in sleep, and are able to note their features?
A: Most probably because they have seen pictures of mountains; otherwise it is somebody or something in us which has previously seen them.
Q: What is the cause of that experience in dreams in which the dreamer seems to be ever striving after something, but never attaining it?
A: It is because the physical self and its memory are shut out of the possibility of knowing what the real Ego does. The dreamer only catches faint glimpses of the doings of the Ego, whose actions produce the so-called dream on the physical man, but is unable to follow it consecutively.
A delirious patient, on recovery, bears the same relation to the nurse who watched and tended him in his illness as the physical man to his real Ego.
The Ego acts as consciously within and without him as the nurse acts in tending and watching over the sick man. But neither the patient after leaving his sick bed nor the dreamer on awaking, will be able to remember anything except in snatches and glimpses.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 