able 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