It may be added that our study of these illusions will help still further to elucidate the exact nature of perception.

Normal mental life, as a whole, at once illustrates, and is illustrated by, abnormal.

And while we need a rough provisional theory of accurate perception in order to explain illusory perception at all, the investigation of this latter cannot fail to verify and even render more complete the theory which it thus temporarily adopts.
_Illusions of Perception._ With this brief psychological analysis of perception to help us, let us now pass to the consideration of the errors incident to the process, with a view to classify them according to their psychological nature and origin.
And here there naturally arises the question, How shall we define an illusion of perception?
When trying to fix the definition of illusion in general, I practically disposed of this question.

Nevertheless, as the point appears to me to be of some importance, I shall reproduce and expand one or two of the considerations then brought forward.
It is said by certain, philosophers that perception, as a whole, is an illusion, inasmuch as it involves the fiction of a real thing independent of mind, yet somehow present to it in the act of sense-perception.

But this is a question for philosophy, not for science.