Illusions | Page 8

to actual phenomena. That there are illusions
of perception is obvious, since it is to the errors of sense that the term
illusion has most frequently been confined. It is hardly less evident that
there are illusions of memory. The peculiar difficulty of distinguishing
between a past real event and a mere phantom of the imagination,
illustrated in the exclamation, "I either saw it or dreamt it," sufficiently
shows that memory is liable to be imposed on. Finally, it is agreed on
by all that the beliefs we are wont to regard as self-evident are
sometimes erroneous. When, for example, an imaginative woman says
she knows, by mere intuition, that something interesting is going to
happen, say the arrival of a favourite friend, she is plainly running the
risk of being self-deluded. So, too, a man's estimate of himself,

however valid for him, may turn out to be flagrantly false.
In the following discussion of the subject I shall depart from the above
order in so far as to set out with illusions of sense-perception. These are
well ascertained, forming, indeed, the best-marked variety. And the
explanation of these has been carried much further than that of the
others. Hence, according to the rule to proceed from the known to the
unknown, there will be an obvious convenience in examining these first
of all. After having done this, we shall be in a position to inquire
whether there is anything analogous in the region of introspection or
internal perception. Our study of the errors of sense-perception will,
moreover, prove the best preparation for an inquiry into the nature and
mode of production of the remaining two varieties.[4]
I would add that, in close connection with the first division, illusions of
perception, I shall treat the subtle and complicated phenomena of
dreams. Although containing elements which ought, according to
strictness, to be brought under one of the other heads, they are, as their
common appellation, "visions," shows, largely simulations of external,
and more especially visual, perception.
Dreams are no doubt sharply marked off from illusions of
sense-perception by a number of special circumstances. Indeed, it may
be thought that they cannot be adequately treated in a work that aims
primarily at investigating the illusions of normal life, and should rather
be left to those who make the pathological side of the subject their
special study. Yet it may, perhaps, be said that in a wide sense dreams
are a feature of normal life. And, however this be, they have quite
enough in common with other illusions of perception to justify us in
dealing with them in close connection with these.
CHAPTER III.
ILLUSIONS OF PERCEPTION: GENERAL.
The errors with which we shall be concerned in this chapter are those
which are commonly denoted by the term illusion, that is to say, those
of sense. They are sometimes called deceptions of the senses; but this is

a somewhat loose expression, suggesting that we can be deceived as to
sensation itself, though, as we shall see later on, this is only true in a
very restricted meaning of the phrase. To speak correctly,
sense-illusions must be said to arise by a simulation of the form of just
and accurate perceptions. Accordingly, we shall most frequently speak
of them as illusions of perception.
In order to investigate the nature of any kind of error, it is needful to
understand the kind of knowledge it imitates, and so we must begin our
inquiry into the nature of illusions of sense by a brief account of the
psychology of perception; and, in doing this, we shall proceed best by
regarding this operation in its most complete form, namely, that of
visual perception.
I may observe that in this analysis of perception I shall endeavour to
keep to known facts, namely, the psychical phenomena or events which
can be seen by the methods of scientific psychology to enter into the
mental content called the percept. I do not now inquire whether such an
analysis can help us to understand all that is meant by perception. This
point will have to be touched later on. Here it is enough to say that,
whatever our philosophy of perception may be, we must accept the
psychological fact that the concrete mental state in the act of perception
is built up out of elements, the history of which can be traced by the
methods of mental science.
Psychology of Perception.
Confining ourselves for the present to the mental, as distinguished from
the physical, side of the operation, we soon find that perception is not
so simple a matter as it might at first seem to be. When a man on a hot
day looks at a running stream and "sees" the delicious coolness, it is not
difficult to show that he is really