It is not even necessary to suppose that the dead man was dreaming about the living person to whom, or about the place in which, he appeared.

But on the analogy of the tales in which a dream or thought of the living seems to produce a hallucination of their presence in the minds of other and distant living people, so a dream of the dead may (it is urged) have a similar effect if "in that sleep of death such dreams may come".

The idea occurred to Shakespeare! In any case the ghosts of our stories hitherto have been so aimless and purposeless as to resemble what we might imagine a dead man's dream to be.
This view of the case (that a "ghost" may be a reflection of a dead man's dream) will become less difficult to understand if we ask ourselves what natural thing most resembles the common idea of a ghost.

You are reading alone at night, let us say, the door opens and a human figure glides into the room.

To you it pays no manner of attention; it does not answer if you speak; it may trifle with some object in the chamber and then steal quietly out again.
_It is the House-maid walking in her Sleep_.
This perfectly accountable appearance, in its aimlessness, its unconsciousness, its irresponsiveness, is undeniably just like the common notion of a ghost.