These remarks at once point out the limits of these illusions.  In normal circumstances, an act of imagination, however vivid, cannot create the semblance of a sensation which is altogether absent; it can only slightly modify the actual impression by interfering with that process of comparison and classification which enters into all definite determination of sensational quality.

Another great fact that has come to light in the investigation of these illusions is that oft-recurring and familiar types of experience leave permanent dispositions in the mind.  As I said when describing the process of perception, what has been frequently perceived is perceived more and more readily.  It follows from this that the mind will be habitually disposed to form the corresponding mental images, and to interpret impressions by help of these.  The range of artistic suggestion depends on this.  A clever draughtsman can indicate a face by a few rough touches, and this is due to the fact that the spectator’s mind is so familiarized, through recurring experience and special interest, with the object, that it is ready to construct the requisite mental image at the slightest external suggestion.  And hence the risk of hasty and illusory interpretation.

These observations naturally conduct us to the consideration of the second great group of sense-illusions, which I have marked off as active illusions, where the action of a pre-existing intellectual disposition becomes much more clearly marked, and assumes the form of a free imaginative transformation of reality.

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.  Thus, for example, in the case just supposed, if another person, bearing some resemblance to our expected friend, chances to come into view, we may probably stumble into the error of taking one person for another.