Does this way of putting the subject seem alarming? Is it an appalling thought that our normal mental life is thus intimately related to insanity, and graduates away into it by such fine transitions? A moment’s reflection will show that the case is not so bad as it seems. It is well to remind ourselves that the brain is a delicately adjusted organ, which very easily gets disturbed, and that the best of us are liable to become the victims of absurd illusion if we habitually allow our imaginations to be overheated, whether by furious passion or by excessive indulgence in the pleasures of day-dreaming, or in the intoxicating mysteries of spiritualist seances. But if we take care to keep our heads cool and avoid unhealthy degrees of mental excitement, we need not be very anxious on the ground of our liability to this kind of error. As I have tried to show, our most frequent illusions are necessarily connected with something exceptional, either in the organism or in the environment. That is to say, it is of the nature of illusion in healthy conditions of body and mind to be something very occasional and relatively unimportant. Our perceptions may be regarded as the reaction of the mind on the impressions borne in from the external world, or as a process of adjustment of internal mental relations to external physical relations. If this process is, in the main, a right one, we need not greatly trouble, because it is not invariably so. We should accept the occasional failure of the intellectual mechanism as an inseparable accompaniment of its general efficiency.
To this it must be added that many of the illusions described above can hardly be called cases of non-adaptation at all, since they have no relation to the practical needs of life, and consequently are, in a general way, unattended to. In other cases, again, namely, where the precise nature of a present sensation, being practically an unimportant matter, is usually unattended to, as in the instantaneous recognition of objects by the eye under changes of illumination, etc., the illusion is rather a part of the process of adaptation, since it is much more important to recognize the permanent object signified by the sensation than the precise nature of the present sensational “sign” itself.
Finally, it should never be forgotten that in normal states of mind there is always the possibility of rectifying an illusion. What distinguishes abnormal from normal mental life is the persistent occupation of the mind by certain ideas, so that there is no room for the salutary corrective effect of reflection on the actual impression of the moment, by which we are wont to “orientate,” or take our bearings as to the position of things about us. In sleep, and in certain artificially produced states, much the same thing presents itself. Images become realities just because they are not instantly recognized as such by a reference to the actual surroundings of the moment. But in normal waking life this power