In both cases there is a reinstatement of the past, a reproduction of earlier experience, a process of adding to a present impression a product of imagination--taking this word in its widest sense.

In both cases the same laws of reproduction or association are illustrated.
Just as a deep and exhaustive analysis of the intellectual operations thus tends to identify their various forms as they are distinguished by the popular mind, so a thorough investigation of the flaws in these operations, that is to say, the counterfeits of knowledge, will probably lead to an identification of the essential mental process which underlies them.

It is apparent, for example, that, whether a man _projects_ some figment of his imagination into the external world, giving it, present material reality, or whether (if I may be allowed the term) he _retrojects_ it into the dim region of the past, and takes it for a reality that has been he is committing substantially the same blunder.

The source of the illusion in both cases is one and the same.
It might seem to follow from this that a scientific discussion of the subject would overlook the obvious distinction between illusions of perception and those of memory; that it would attend simply to differences in the mode of origination of the illusion, whatever its external form.

Our next step, then, would appear to be to determine these differences in the mode of production.
That there are differences in the origin and source of illusion is a fact which has been fully recognized by those writers who have made a special study of sense-illusions.