But if the sensation is properly attended to, can there be error through a misapprehension of what is actually in the mind at the moment?  To say that there can may sound paradoxical, and yet in a sense this is demonstrable.  I do not mean that there is an observant mind behind and distinct from the sensation, and failing to observe it accurately through a kind of mental short-sightedness.  What I mean is that the usual psychical effect of the incoming nervous process may to some extent be counteracted by a powerful reaction of the centres.  In the course of our study of illusions, we shall learn that it is possible for the quality of an impression, as, for example, of a sensation of colour, to be appreciably modified when there is a strong tendency to regard it in one particular way.

Postponing the consideration of these, we may say that certain illusions appear clearly to take their start from an error in the process of classifying or identifying a present impression.  On the physical side, we may say that the first stages of the nervous process, the due excitation of the sensory centre in accordance with the form of the incoming stimulation and the central reaction involved in the recognition of the sensation, are incomplete.  These are so limited and comparatively unimportant a class, that it will be well to dispose of them at once.

Confusion of the Sense-Impression.

The most interesting case of such an error is where the impression is unfamiliar and novel in character.  I have already remarked that in the mental life of the adult perfectly new sensations never occur.  At the same time, comparatively novel impressions sometimes arise.  Parts of the sensitive surface of the body which rarely undergo stimulation are sometimes acted on, and at other times they receive partially new modes of stimulation.  In such cases it is plain that the process of classing the sensation or recognizing it is not completed.  It is found that whenever this happens there is a tendency to exaggerate the intensity of the sensation.  The very fact of unfamiliarity seems to give to the sensation a certain exciting character.  As something new and strange, it for the instant slightly agitates and discomposes the mind.  Being unable to classify it with its like, we naturally magnify its intensity, and so tend to ascribe it to a disproportionately large cause.

For instance, a light bandage worn about the body at a part usually free from pressure is liable to be conceived as a weighty mass.  The odd sense of a big cavity in the mouth, which we experience just after the loss of a tooth, is probably another illustration of this principle.  And a third example may also be supplied from the recollection of the dentist’s patient, namely, the absurd imagination which he tends to form as to what is actually going on in his mouth when a tooth is being bored by a modern rotating drill.  It may be found that the same principle helps to account for the exaggerated importance which we attach to the impressions of our dreams.