CHAPTER IV.
ILLUSIONS OF PERCEPTION--_continued_.
A._Passive Illusions (a) as determined by the Organism._ In dealing with the illusions which are related to certain peculiarities in the nervous organism and the laws of sensibility, I shall commence with those which are connected with certain limits of sensibility.
_Limits of Sensibility._ To begin with, it is known that the sensation does not always answer to the external stimulus in its degree or intensity.

Thus, a certain amount of stimulation is necessary before any sensation arises.

And this will, of course, be greater when there is little or no attention directed to the impression, that is to say, no co-operating central reaction.

Thus it happens that slight stimuli go overlooked, and here illusion may have its starting-point.

The most familiar example of such slight errors is that of movement.