Mark Twain, who is seriously interested in these subjects, has
published an experience illustrative of such possibilities. He tells
his tale at considerable length, but it amounts to this:--

MARK TWAIN'S STORY

Mark was smoking his cigar outside the door of his house when he saw a
man, a stranger, approaching him. Suddenly he ceased to be visible!
Mark, who had long desired to see a ghost, rushed into his house to
record the phenomenon. There, seated on a chair in the hall, was the
very man, who had come on some business. As Mark's negro footman
acts, when the bell is rung, on the principle, "Perhaps they won't
persevere," his master is wholly unable to account for the
disappearance of the visitor, whom he never saw passing him or waiting
at his door--except on the theory of an unconscious nap. Now, a
disappearance is quite as mystical as an appearance, and much less
common.

This theory, that apparitions come in an infinitesimal moment of
sleep, while a man is conscious of his surroundings and believes
himself to be awake was the current explanation of ghosts in the
eighteenth century. Any educated man who "saw a ghost" or "had a
hallucination" called it a "dream," as Lord Brougham and Lord
Lyttelton did. But, if the death of the person seen coincided with
his appearance to them, they illogically argued that, out of the
innumerable multitude of dreams, some _must_ coincide, accidentally,
with facts. They strove to forget that though dreams in sleep are
universal and countless, "dreams" in waking hours are extremely rare--
unique, for instance, in Lord Brougham's own experience. Therefore,