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.