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.