Not only does our idea of the past become inexact by the mere decay and disappearance of essential features, it becomes positively incorrect through the gradual incorporation of elements that do not properly belong to it.  Sometimes it is easy to see how these extraneous ideas get imported into our mental representation of a past event.  Suppose, for example, that a man has lost a valuable scarf-pin.  His wife suggests that a particular servant, whose reputation does not stand too high, has stolen it.  When he afterwards recalls the loss, the chances are that he will confuse the fact with the conjecture attached to it, and say he remembers that this particular servant did steal the pin.  Thus, the past activity of imagination serves to corrupt and partially falsify recollections that have a genuine basis of fact.

It is evident that this class of mnemonic illusions approximates in character to illusions of perception.  When the imagination supplies the interpretation at the very time, and the mind reads this into the perceived object, the error is one of perception.  When the addition is made afterwards, on reflecting upon the perception, the error is one of memory.  The “fallacies of testimony” which depend on an adulteration of pure observation with inference and conjecture, as, for example, the inaccurate and wild statements of people respecting their experiences at spiritualist seances, while they illustrate the curious blending of both kinds of error, are probably much oftener illusions of memory than of perception.[125]

Although in many cases we can account to ourselves for this confusion of fact and imagination, in other cases it is difficult to see any close relation between the fact remembered and the foreign element imported into it.  An idea of memory seems sometimes to lose its proper moorings, so to speak; to drift about helplessly among other ideas, and finally, by some chance, to hook itself on to one of these, as though it naturally belonged to it.  Anybody who has had an opportunity of carefully testing the truthfulness of his recollection of some remote event in early life will have found how oddly extraneous elements become incorporated into the memorial picture.  Incidents get put into wrong places, the wrong persons are introduced into a scene, and so on.  Here again we may illustrate the mnemonic illusion by a visual one.  When a tree standing before or behind a house and projecting above or to the side of it is not sharply distinguished from the latter, it may serve to give it a very odd appearance.

These confusions of the mental image may arise even when only a short interval has elapsed.  In the case of many of the fleeting impressions that are only half recollected, this kind of error is very easy.  Thus, for example, I may have lent a book to a friend last week.  I really remember the act of lending it, but have forgotten the person.  But I am not aware of this.  The picture of memory has unknowingly