ily reduced to classes, each of which may be
referred to its peculiar organ of sensation, because each organ is so
constituted, that it can only be affected by one class of properties;
thus the eye can only be affected by light; the ear by the vibrations
of the air, and so of the rest.

The same organ, whatever be its state, or whatever be the degree of
impulse, always gives to the mind a similar sensation; nor is it
possible, by any means we are acquainted with, to communicate the
sensation peculiar to one organ by means of another. Thus we are
incapable, for instance, of hearing with our eyes, and seeing with
our ears: nor have we any reason to believe that similar impressions
produce dissimilar sensations in different people. The pleasure,
however, as well as the pain and disgust, accompanying different
sensations, differ very greatly in different persons, and even in the
same person at different times; for the sensations which sometimes
afford us pleasure, at other times produce disgust.

Habit has a powerful influence in modifying the pleasures of
sensation, without producing any change in the sensation itself, or
in the external qualities suggested by it. Habit, for instance, will
never cause a person to mistake gentian or quassia for sugar, but it
may induce an appetite or liking for what is bitter, and a disgust
for what is sweet. No person perhaps was originally delighted with
the taste of opium or tobacco, they must at first be highly
disgusting to most people; but custom not only reconciles the taste
to them, but they become grateful, and even necessary.

Almost every species of sensation becomes grateful or otherwise,
according to the force of the impression; for there is no sensation
so pleasant, but, that, by increasing its intensity, it will become
ungrateful, and at length intolerable. And, on the contrary, there
are many which on account of their force are naturally unpleasant,
but become, when diminished, highly pleasant. The softest and
sweetest sounds may be increased to such a degree as to be extremely
unpleasant: and when we are in the steeple of a church, the noise of
a peal of bells stuns and confounds our senses, while at a distance
their effect is very pleasant. The smell of musk likewise at a
distance, and in small quantity, is pleasant; but when brought near,
or in large quantity, it becomes highly disagreable. The same may be
observed with respect to the objects of the other senses.

Fo