On the other hand, things merely imagined may sometimes be very vivid
and insistent. To some persons, what exists in the imagination is dim
and indefinite in the extreme. Others imagine things vividly, and can
describe what is present only to the imagination almost as though it
were something seen. Finally, we know that an image may become so
vivid and insistent as to be mistaken for an external thing. That is
to say, there are such things as hallucinations.

The criterion of vividness will not, therefore, always serve to
distinguish between what is given in the sense and what is only
imagined. And, indeed, it becomes evident, upon reflection, that we do
not actually make it our ultimate test. We may be quite willing to
admit that faint sensations may come to be confused with what is
imagined, with "ideas," but we always regard such a confusion as
somebody's error. We are not ready to admit that things perceived
faintly are things imagined, or that vivid "ideas" are things perceived
by sense.

Let us come back to the illustrations with which we started. How do I
know that I perceive the desk before me; and how do I know that,
sitting here, I imagine, and do not see, the front door of the house?

My criterion is this: when I have the experience I call "seeing my
desk," the bit of experience which presents itself as my desk is in a
certain setting. That is to say, the desk seen must be in a certain
relation to my body, and this body, as I know it, also consists of
experiences. Thus, if I am to know that I see the desk, I must realize
that my eyes are open, that the object is in front of me and not behind
me, etc.