To the psychologist it comes to very much the same thing whether, for
example, on a visit to Switzerland, our minds are occupied in
_perceiving_ the distance of a mountain or in _remembering_ some
pleasant excursion which we made to it on a former visit. 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