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.