This great source of error has been so abundantly illustrated under the head of Passive Illusions that I need not dwell on it further.  It is plain that a passive error of perception, or of expectation, is due in general to a defective grouping of elements, to a grouping which answers, perhaps, to the run of the individual’s actual experience, but not to a large and complete common experience.[148] Similarly, an illusory general belief is plainly a welding together of elements (here concepts, answering to innumerable representative images) in disagreement with the permanent connections of experience.  Even a passive illusion of memory, in so far as it involves a rearrangement of successive representations, shows the same kind of defect.

In the second place, this incorrect grouping maybe due, not to defects in attention and discrimination, combined with insufficiently grounded association, but to the independent play of constructive imagination and the caprices of feeling.  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.  This holds good even of the simplest kinds of error in which a presentative element is wrongly classed; and it holds good of those more conspicuous errors of perception, memory, expectation, and compound belief, in which representations connect themselves in an order not perfectly answering to the objective order.

This view of the nature and causes of illusion is clearly capable of being expressed in physical language.  Bad grouping of psychical elements is equivalent to imperfect co-ordination of their physical, that is to say, nervous, conditions, imperfect in the evolutionist’s sense, as not exactly according with external relations.  So far as illusions of suggestion (passive illusions) are concerned, the error is connected with organized tendencies, due to a limited action of experience.  On the other hand, illusions of preconception (active illusions) usually involve no such deeply fixed or permanent organic connections, but merely a temporary confluence of nerve-processes.[149] The nature of the physical process is best studied in the case of errors of sense-perception.  Yet we may hypothetically argue that even in the case of the most complex errors, as those of memory and of belief, there is implied a deviation in the mode of connection of nervous structures (whether the connection be permanent or temporary) from the external order of facts.