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.