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 mediaeval 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.