In a wide sense, again, all errors connected with those subjective sensations which arise from a stimulation of the peripheral regions of the nerve may be called illusions rather than hallucinations.  Or, if they must be called hallucinations, they may be distinguished as “peripheral” from those “central” hallucinations which arise through an internal automatic excitation of the sensory centre.  It is plain from this that the region of subjective sensation is an ambiguous region, where illusion and hallucination mix and become confused.  To this point I shall have occasion to return by-and-by.

I have now probably said enough respecting the illusions that arise through the fact of there being fixed limits to our sensibility.  The rationale of these illusions is that whenever the limit is reached, we tend to ignore it and to interpret the impression in the customary way.

Variations of Sensibility.

We will now pass to a number of illusions which depend on something variable in the condition of our sensibility, or some more or less exceptional organic circumstance.  These variations may be momentary and transient or comparatively permanent.  The illusion arises in each case from our ignoring the variation, and treating a given sensation under all circumstances as answering to one objective cause.

First of all, the variation of organic state may affect our mental representation of the strength of the stimulus or external cause.  Here the fluctuation may be a temporary or a permanent one.  The first case is illustrated in the familiar example of taking a room to be brighter than it is when emerging from a dark one.  Another striking example is that of our sense of the temperature of objects, which is known to be strictly relative to a previous sensation, or more correctly to the momentary condition of the organ.  Yet, though every intelligent person knows this, the deeply rooted habit of making sensation the measure of objective quality asserts its sway, and frequently leads us into illusion.  The well-known experiment of first plunging one hand in cold water, the other in hot, and then dipping them both in tepid, is a startling example of this organized tendency.  For here we are strongly disposed to accept the palpable contradiction that the same water is at once warm and cool.

Far more important than these temporary fluctuations of sensibility are the permanent alterations.  Excessive fatigue, want of proper nutrition, and certain poisons are well known to be causes of such changes.  They appear most commonly under two forms, exalted sensibility, or hyperaesthesia, and depressed sensibility, or anaesthesia.  In these conditions flagrant errors are made as to the real magnitude of the causes of the sensations.  These variations may occur in normal life to some extent.  In fairly good health we experience at times strange exaltations of tactual sensibility, so that a very slight stimulus, such as the contact of the bed-clothes, becomes greatly exaggerated.