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
hyperæsthesia, and depressed sensibility, or anæsthesia. 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.

In diseased states of the nervous system these variations of sensibility
become much more striking. The patient who has hyperæsthesia fears to
touch a perfectly smooth surface, or he takes a knock at the door to be
a clap of thunder. The hypochondriac may, through an increase of
organic sensibility, translate organic sensations as the effect of some
living creature gnawing at his vitals. Again, states of anæsthesia lead
to odd illusions among the insane. The common supposition that the body
is dead, or made of wood or of glass, is clearly referable in part to
lowered sensibility of the organism.[32]