If illusion is the simulation of immediate knowledge, the most obvious mode of classifying illusions would appear to be according to the variety of the knowledge which they simulate. Now, the popular psychology that floats about in the ordinary forms of language has long since distinguished certain kinds of unreasoned or uninferred knowledge. Of these the two best known are perception and memory. When I see an object before me, or when I recall an event in my past experience, I am supposed to grasp a piece of knowledge directly, to know something immediately, and not through the medium of something else. Yet I know differently in the two cases. In the first I know by what is called a presentative process, namely, that of sense-perception; in the second I know by a representative process, namely, that of reproduction, or on the evidence of memory. In the one case the object of cognition is present to my perceptive faculties; in the other it is recalled by the power of memory. 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.