In common life, we apply the test instinctively, and with little realization of what we are doing.
And if we turn to the psychologist, whose business it is to be more exact and scientific, we find that he gives us only a refinement of this same criterion.

It is important to him to distinguish between what is given in sensation and what is furnished by memory or imagination, and he tells us that sensation is the result of a message conducted along a sensory nerve to the brain.
Here we see emphasized the relation to the body which has been mentioned above.

If we ask the psychologist how he knows that the body he is talking about is a real body, and not merely an imagined one, he has to fall back upon the test which is common to us all.

A real hand is one which we see with the eyes open, and which we touch with the other hand.

If our experiences of our own body had not the setting which marks all sensory experiences, we could never say: I _perceive_ that my body is near the desk.