is 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
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