hall soon see, an important co-operant condition of
mnemonic illusion, but does not constitute it, any more than haziness of
vision or disease of the visual organ, though highly favourable to
optical illusion, can be said to constitute it.

We may conveniently proceed in our detailed examination of illusions of
memory, by distinguishing between three facts which appear to be
involved in every complete and accurate process of recollection. When I
distinctly recall an event, I am immediately sure of three things: (1)
that something did really happen to me; (2) that it happened in the way
I now think; and (3) that it happened when it appears to have happened.
I cannot be said to recall a past event unless I feel sure on each of
these points. Thus, to be able to say that an event happened at a
particular date, and yet unable to describe how it happened, means that
I have a very incomplete recollection. The same is true when I can
recall an event pretty distinctly, but fail to assign it its proper
date. This being so, it follows that there are three possible openings,
and only three, by which errors of memory may creep in. And, as a matter
of fact, each of these openings will be found to let in one class of
mnemonic illusion. Thus we have (1) false recollections, to which there
correspond no real events of personal history; (2) others which
misrepresent the manner of happening of the events; and (3) others which
falsify the date of the events remembered.

It is obvious, from a mere glance at this threefold classification, that
illusions of memory closely correspond to visual illusions. Thus, class
(1) may be likened to the optical illusions known as subjective
sensations of light, or ocular spectra. Here we can prove that there is
nothing actually seen in the field of vision, and that the semblance of
a visible object arises from quite another source than that of ordinary
external light-stimulation, and by what may be called an accident.
Similarly, in the case of the first class of mnemonic illusions, we
shall find that there is nothing actually recollected, but that the
mnemonic spectra or phantoms of recollected objects can be accounted for
in quite another way. 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 whi