This being so, we may very roughly describe all illusion as abnormal.

Just as hallucination, the most signal instance of illusion, is distinctly on the border-land of healthy and unhealthy mental life; just as dreams are in the direction of such unhealthy mental action; so the lesser illusions of memory and so on are abnormal in the sense that they imply a departure from a common typical mode of intellectual action.
It is plain, indeed, that this is the position we have been, taking up throughout our discussion of illusion.

We have assumed that what is common and normal is true, or answers to what is objectively real.

Thus, in dealing with errors of perception, we took for granted that the common percept--meaning by this what is permanent in the individual and the general experience--is at the same time the true percept.

So in discussing the illusions of memory we estimated objective time by the judgment of the average man, free from individual bias, and apart from special circumstances favourable to error.