CHAPTER I.

THE STUDY OF ILLUSION.


Common sense, knowing nothing of fine distinctions, is wont to draw a
sharp line between the region of illusion and that of sane intelligence.
To be the victim of an illusion is, in the popular judgment, to be
excluded from the category of rational men. The term at once calls up
images of stunted figures with ill-developed brains, half-witted
creatures, hardly distinguishable from the admittedly insane. And this
way of thinking of illusion and its subjects is strengthened by one of
the characteristic sentiments of our age. The nineteenth century
intelligence plumes itself on having got at the bottom of mediæval
visions and church miracles, and it is wont to commiserate the feeble
minds that are still subject to these self-deceptions.

According to this view, illusion is something essentially abnormal and
allied to insanity. And it would seem to follow that its nature and
origin can be best studied by those whose speciality it is to observe
the phenomena of abnormal life. Scientific procedure has in the main
conformed to this distinction of common sense. The phenomena of illusion
have ordinarily been investigated by alienists, that is to say,
physicians who are brought face to face with their most striking forms
in the mentally deranged.

While there are very good reasons for this treatment of illusion as a
branch of mental pathology, it is by no means certain that it can be a