Thus, all our sensory experiences are experiences that fall into a certain system or order.  It is a system which we all recognize implicitly, for we all reject as merely imaginary those experiences which lack this setting.  If my eyes are shut—­I am speaking now of the eyes as experienced, as felt or perceived, as given in sensation—­I never say; “I see my desk,” no matter how vivid the image of the object.  Those who believe in “second sight” sometimes talk of seeing things not in this setting, but the very name they give to the supposed experience indicates that there is something abnormal about it.  No one thinks it remarkable that I see the desk before which I perceive myself to be sitting with open eyes.  Every one would think it strange if I could see and describe the table in the next room, now shut away from me.  When a man thinks he hears his name pronounced, and, turning his head, seeks in vain for the speaker, he sets his experience down as a hallucination.  He says, I did not really hear that; I merely imagined it.

May one not, with open eyes, have a hallucination of vision, just as one may seem to hear one’s name pronounced when no one is by?  Certainly.  But in each case the experience may be proved to be a hallucination, nevertheless.  It may be recognized that the sensory setting is incomplete, though it may not, at first, seem so.  Thus the unreal object which seems to be seen may be found to be a thing that cannot be touched.  Or, when one has attained to a relatively complete knowledge of the system of experiences recognized as sensory, one may make use of roundabout methods of ascertaining that the experience in question does not really have the right setting.  Thus, the ghost which is seen by the terrified peasant at midnight, but which cannot be photographed, we may unhesitatingly set down as something imagined and not really seen.

All our sensations are, therefore, experiences which take their place in a certain setting.  This is our ultimate criterion.  We need not take the word of the philosopher for it.  We need only reflect, and ask ourselves how we know that, in a given case, we are seeing or hearing or touching something, and are not merely imagining it.  In every case, we shall find that we come back to the same test.  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.