In this way the belief in demoniacal possession (distinguished, as such, from madness and epilepsy) has its nucleus, some contend, in the phenomena of alternating personalities in certain patients.

Their characters, ideas, habits, and even voices change, and the most obvious solution of the problem, in the past, was to suppose that a new alien personality--a "devil"-- had entered into the sufferer.
Again, the phenomena occurring in "haunted houses" (whether caused, or not, by imposture or hallucination, or both) were easily magnified into such legends as that of Grettir and Glam, and into the monstrosities of the witch trials.

Once more the simple hallucination of a dead person's appearance in his house demanded an explanation.
This was easily given by evolving a legend that he was a spirit, escaped from purgatory or the grave, to fulfil a definite purpose.
The rarity of such purposeful ghosts in an age like ours, so rich in ghost stories, must have a cause.

That cause is, probably, a dwindling of the myth-making faculty.
Any one who takes these matters seriously, as facts in human nature, must have discovered the difficulty of getting evidence at first hand.
This arises from several causes.

First, the cock-sure common-sense of the years from 1660 to 1850, or so, regarded every one who had experience of a hallucination as a dupe, a lunatic, or a liar.