Such illusions come nearest to hallucinations in the region of memory.
Again, class (2) has its visual analogue in those optical illusions which depend on effects of haziness and of the action of refracting media interposed between the eye and the object; in which cases, though there is some real thing corresponding to the perception, this is seen in a highly defective, distorted, and misleading form.

In like manner, we can say that the images of memory often get obscured, distorted, and otherwise altered when they have receded into the dim distance, and are looked back upon through a long space of intervening mental experience.
Finally, class (3) has its visual counterpart in erroneous perceptions of distance, as when, for example, owing to the clearness of the mountain atmosphere and the absence of intervening objects, the side of the Jungfrau looks to the inexperienced tourist at Wengernalp hardly further than a stone's throw.

It will be found that when our memory falsifies the date of an event, the error arises much in the same way as a visual miscalculation of distance.
This threefold division of illusions of memory is plainly a rather superficial one, and not based on distinctions of psychological nature or origin.

In order to make our treatment of the subject scientific as well as popular, it will be necessary to introduce the distinction between the passive and the active factor under each head.

It will be found, I think, without forcing the analogy too far, that here, as in the case of the illusions of perception and introspection, error is attributable now to misleading suggestion on the part of the mental content of the moment, now to a process of incorporating into this content a mental image not suggested by it, but existing independently.
If we are to proceed as we did in the case of the illusions of sense, and take up the lower stages of error first of all, we shall need to begin with the third class of errors, those of localization in time, or of what may be called mnemonic perspective.