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.