In our waking state, when our powers of volition are intact, the external impression is characterized by its fixity or its obdurate resistance to our wishes.

On the other hand, the mental image is fluent, accommodating, and disappears and reappears according to the direction of our volitions.

In sleep, through the suspension of the higher voluntary power of attention, the mental image seems to lord it over our minds just as the actual impression of waking life.
This much may suffice, perhaps, by way of a general description of the sleeping and dreaming state.

Other points will make themselves known after we have studied the contents and structure of dreams in detail.
Dreams are commonly classified (_e.g._ by Wundt) with hallucinations, and this rightly, since, as their common appellation of "vision" suggests, they are for the most part the semblance of percepts in the absence of external impressions.

At the same time, recent research goes to show that in many dreams something answering to the "external impression" in waking perception is the starting-point.