To forget or be doubtful about a past event is one thing; to seem to ourselves to remember it when we afterwards find that the fact was otherwise than we represented it in the apparent act of recollection is another thing.

Indistinctness of recollection, or the decay of memory, is, as we shall 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.