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.  Speaking broadly, one may describe all knowledge as a correspondence of representation with fact or experience, or as a stable condition of the representation which cannot be disturbed by new experiences.  It does not matter, for our present purpose, whether the fact represented is supposed to be directly present, as in presentative cognition; or to be absent, either as something past or future, or finally as a “general fact,” that is to say, the group of facts (past and future) embodied in a universal proposition.[147]

In general this accordance between our representations and facts is secured by the laws of our intellectual mechanism.  It follows from the principles of association that our simple experiences, external and internal, will tend to reflect themselves in perception, memory, expectation, and general belief, in the very time-connections in which they actually occur.  To put it briefly, facts which occur together will in general be represented together, and they will be the more perfectly co-represented in proportion to the frequency of this concurrence.

Illusion, as distinguished from correct knowledge, is, to put it broadly, deviation of representation from fact.  This is due in part to limitations and defects in the intellectual mechanism itself, such as the imperfections of the activities of attention, discrimination, and comparison, in relation to what is present.  Still more is it due to the control of our mental processes by association and habit.  These forces, which are at the very root of intelligence, are also, in a sense, the originators of error.  Through the accidents of our experience or the momentary condition of our reproductive power, representations get wrongly grouped with presentations and with one another; wrongly grouped, that is to say, according to a perfect or ideal standard, namely, that the grouping should always exactly agree with the order of experience as a whole, and the force of cohesion be proportionate to the number of the conjunctions of this experience.