Illusions | Page 5

or appear to do so (for we must not yet assume that there are such) after
a certain kind of philosophic reflection. And some attempt will be made
to determine roughly how far the process of dissolving these substantial
beliefs of mankind into airy phantasms may venture to go.
For the present, however, these so-called illusions in philosophy will be
ignored. It is plain that illusion exists only in antithesis to real
knowledge. This last must be assumed as something above all question.
And a rough and provisional, though for our purpose sufficiently
accurate, demarcation of the regions of the real and the illusory seems
to coincide with the line which common sense draws between what all
normal men agree in holding and what the individual holds, whether
temporarily or permanently, in contradiction to this. For our present
purpose the real is that which is true for all. Thus, though physical
science may tell us that there is nothing corresponding to our sensations
of colour in the world of matter and motion which it conceives as
surrounding us; yet, inasmuch as to all men endowed with the normal
colour-sense the same material objects appear to have the same colour,
we may speak of any such perception as practically true, marking it off
from those plainly illusory perceptions which are due to some
subjective cause, as, for example, fatigue of the retina.
To sum up: in treating of illusions we shall assume, what science as
distinguished from philosophy is bound to assume, namely, that human
experience is consistent; that men's perceptions and beliefs fall into a

consensus. From this point of view illusion is seen to arise through
some exceptional feature in the situation or condition of the individual,
which, for the time, breaks the chain of intellectual solidarity which
under ordinary circumstances binds the single member to the collective
body. Whether the common experience which men thus obtain is
rightly interpreted is a question which does not concern us here. For
our present purpose, which is the determination and explanation of
illusion as popularly understood, it is sufficient that there is this general
consensus of belief, and this may provisionally be regarded as at least
practically true.
CHAPTER II.
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. 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