THE CLASSIFICATION OF ILLUSIONS.


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.