In many cases, indeed, as, for example, in looking at a reflection in a mirror, the illusion is very imperfect, remaining in the nascent stage.
Again, a little attention to the facts here brought together will show that the proportion of illusory to real knowledge is far from being the same in each class of immediate or quasi-immediate cognition.

Thus, with respect to the great distinction between presentative and representative knowledge, it is to be observed that, in so far as any act of cognition is, strictly speaking, presentative, it does not appear to admit of error.

The illusions of perception are connected with the representative side of the process, and are numerous just because this is so extensive.
On the other hand, in introspection, where the scope of independent representation is so limited, the amount of illusion is very inconsiderable, and may in practice be disregarded.

So again, to take a narrower group of illusions, we find that in the recalling of distant events the proportion of error is vastly greater than in the recalling of near events.
So much as to the extent of illusion as brought to light by our preceding study.

Let us now glance at the conclusions obtained respecting its nature and its causes.
_Causes of Illusion._ Looking at illusion as a whole, and abstracting from the differences of mental mechanism in the processes of perception, memory, etc., we may say that the _rationale_ or mode of genesis of illusion is very much the same throughout.