CHAPTER VI.
ILLUSIONS OF PERCEPTION--_continued_.
B._Active Illusions._ When giving an account of the mechanism of perception, I spoke of an independent action of the imagination which tends to anticipate the process of suggestion from without.

Thus, when expecting a particular friend, I recognize his form much more readily than when my mind has not been preoccupied with his image.
A little consideration will show that this process must be highly favourable to illusion.

To begin with, even if the preperception be correct, that is to say, if it answer to the perception, the mere fact of vivid expectation will affect the exact moment of the completed act of perception.

And recent experiment shows that in certain cases such a previous activity of expectant attention may even lead to the illusory belief that the perception takes place before it actually does.[47] A more palpable source of error resides in the risk of the formation of an inappropriate preperception.

If a wrong mental image happens to have been formed and vividly entertained, and if the actual impression fits in to a certain extent with this independently formed preperception, we may have a fusion of the two which exactly simulates the form of a complete percept.