ade. Next they supposed, that the action of the will
caused a motion of the ether to be instantly propagated along the
nerves that terminate in the fibres of the muscles, which
stimulated them to contraction.

Other philosophers imagined, that a tremulous motion was excited
in the nerves themselves, by the action of external impulses, like
the motions excited in the string of a harp. These motions they
supposed to be propagated along the nerves of sense, to the brain,
and from thence along the motory nerves, to the muscles.

Before they attempted this explanation of the phenomena, they
should have proved the existence of such a fluid, or at least
brought forward such circumstances, as rendered its existence
credible. But supposing we grant them the hypothesis, it will, in
my opinion, not avail much; for it is not easy to conceive how the
motion of a subtile fluid, or the vibration of a nerve, can cause
sensation.

Nor are the internal senses, as they are generally called, namely,
memory, and imagination, any better explained on this supposition;
for we cannot conceive how this nervous fluid is stored up and
propelled by the will.

After all, I think we must confess, that this subject is still
enveloped in obscurity. One observation is worth making, namely,
that our sensations have not the smallest resemblance to the
substance or impression, which causes them; thus the sensation
occasioned by the smell of camphor, possesses not the smallest
resemblance to small particles of camphor floating in the
atmosphere; a sensation of pain has no similitude whatever to the
point of a sword which occasions it; nor has the sensation of
sound any resemblance to waves or tremors in the air. In our
present state of knowledge, therefore, all that we can say, is,
that nature has so formed us, that when an impression is made on
any of the organs of sense, it causes a sensation, which forces us
to believe in the existence of an external object, though we
cannot see any connexion between the sensation and the object
which produces it.

But though philosophers were certainly blameable for assuming an
unknown cause, to account for various phenomena, yet later
experiments would seem to prove that even the conjectures of
Newton were not founded on slight grounds. His idea that the
diamond was inflammable, has been confirmed by various
experiments: and that there exists in nature a subtile fluid,
capable of pervading with ease the denses