ion has generally been defined by metaphysicians to be a change
in the mind, of which we are conscious, caused by a correspondent
change in the state of the body. This definition, however, leaves the
matter where they found it, and throws no light whatever on the
nature of sensation; nor can we say any thing more concerning it,
than that, when the organs are in a sound state, certain sensations
are perceived, which force us to believe in the existence of external
objects, though there is no similarity whatever, nor any necessary
connexion, that we can perceive, between the sensation and the object
which caused it.

All the different degrees of sensation may be reduced to two kinds:
pleasant and painful. The nature of these two primitive modes of
sensation, is as little known to us as their different species: all
that can be said, is, that the general laws by which the body is
governed, are such, that pleasure is generally connected with those
impressions which tend to its preservation, and pain with those which
cause its destruction.

In a general point of view, sensibility may be regarded as an
essential property of every part of the living body, disposing each
part to perform those functions, the object of which is to preserve
the life of the animal. Sensibility presides over the most necessary
functions, and watches carefully over the health of the body: she
directs the choice of the air proper for respiration, and also of
alimentary substances; the mechanism of the secretions is likewise
placed under her power; and in the same way that the eye perceives
colours, and the ear sounds, so every animated and living part is
fitted to receive impressions from the objects appropriated to it.

That every part of the animal is endowed with sensibility, is evident
from a variety of facts, particularly from the action which follows
when a muscle taken out of the animal body is irritated by any
stimulus: this is evident, by a variety of facts mentioned by Whytt,
Boerhaave, and others, which show, that parts recently taken from the
animal body retain a portion of sensibility, which continues to
animate them, and render them capable of action for a considerable
time.

The primary organ of sensation appears to be the brain, its
continuation in the form of medulla oblongata and spinal marrow, and
the various nerves proceeding from these; and it seems now generally
agreed, that unless there be a free communication of nerves betw