Scientific psychology tends, no doubt, to break down some of these popular distinctions.  Just as the zoologist sometimes groups together varieties of animals which the unscientific eye would never think of connecting, so the psychologist may analyze mental operations which appear widely dissimilar to the popular mind, and reduce them to one fundamental process.  Thus recent psychology draws no sharp distinction between perception and recollection.  It finds in both very much the same elements, though combined in a different way.  Strictly speaking, indeed, perception must be defined as a presentative-representative operation.  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 special study of sense-illusions.  By these the term illusion is commonly employed in a narrow, technical sense, and opposed to hallucination.  An illusion, it is said, must always have its starting-point in some actual impression, whereas a hallucination has no such basis.  Thus it is an illusion when a man, under the action of terror, takes a stump of a tree, whitened by the moon’s rays, for a ghost.  It is a hallucination when an imaginative person so vividly pictures to himself the form of some absent friend that, for the moment, he fancies himself actually beholding him.  Illusion is thus a partial displacement of external fact by a fiction of the imagination, while hallucination is a total displacement.