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.