I say, as a rule, for we cannot always remark this difference.  Sensations may be very clear and unmistakable, but they may also be very faint and indefinite.  When a man lays his hand firmly on my shoulder, I may be in little doubt whether I feel a sensation or do not; but when he touches my back very lightly, I may easily be in doubt, and may ask myself in perplexity whether I have really been touched or whether I have merely imagined it.  As a vessel recedes and becomes a mere speck upon the horizon, I may well wonder, before I feel sure that it is really quite out of sight, whether I still see the dim little point, or whether I merely imagine that I see it.

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.

The desk as seen varies with the relation to the body in certain ways that we regard as natural and explicable.  When I am near it, the visual experience is not just what it is when I recede from it.  But how can I know that I am near the desk or far from it?  What do these expressions mean?  Their full meaning will become clearer in the next chapter, but here I may say that nearness and remoteness must be measured for me in experiences of some sort, or I would never know anything as near to or far from my body.