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 performing an act of mental
synthesis, or imaginative construction. To the sense-impression[5] which
his eye now gives him, he adds something which past experience has
bequeathed to his mind. In perception, the material of sensation is
acted on by the mind, which embodies in its present attitude all the
results of its past growth. Let us look at this process of synthesis a
little more closely.

When a sensation arises in the mind, it may, under certain
circumstances, go unattended to. In that case there is no perception.
The sensation floats in the dim outer regions of consciousness as a
vague feeling, the real nature and history of which are