This is illustrated in what I have called Active Illusions, whether the excited perceptions and the hallucinations of sense, or the fanciful projections of memory or of expectation.

Here we have a force directly opposed to that of experience.

Active illusion arises, not through the imperfections of the intellectual mechanism, but through a palpable interference with this mechanism.

It is a regrouping of elements which simulates the form of a suggestion by experience, but is, in reality, the outcome of the individual mind's extra-intellectual impulses.
We see, then, that, in spite of obvious differences in the form, the process in all kinds of immediate cognition is fundamentally identical.
It is essentially a bringing together of elements, whether similar or dissimilar and associated by a link of contiguity, and a viewing of these as connected parts, of a whole; it is a process of synthesis.

And illusion, in all its forms, is bad grouping or carelessly performed synthesis.