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