The senses of touch and of hearing acquaint us with the mechanical impact
and vibration of bodies, those of smell and taste seem to acquaint us with
some of their chemical properties, while the sense of vision and of heat
acquaint us with the existence of their peculiar fluids.

_Sensation and Volition._

II. Many motions are produced by pleasure or pain, and that even in
contradiction to the power of volition, as in laughing, or in the
strangury; but as no name has been given to pleasure or pain, at the time
it is exerted so as to cause fibrous motions, we have used the term
sensation for this purpose; and mean it to bear the same analogy to
pleasure and pain, that the word volition does to desire and aversion.

1. It was mentioned in the fifth Section, that, what we have termed
sensation is a motion of the central parts, or of the whole sensorium,
_beginning_ at some of the extremities of it. This appears first, because
our pains and pleasures are always caused by our ideas or muscular motions,
which are the motions of the extremities of the sensorium. And, secondly,
because the sensation of pleasure or pain frequently continues some time
after the ideas or muscular motions which excited it have ceased: for we
often feel a glow of pleasure from an agreeable reverie, for many minutes
after the ideas, that were the subject of it, have escaped our memory; and
frequently experience a dejection of spirits without being able to assign
the cause of it but by much recollection.

When the sensorial faculty of desire or aversion is exerted so as to cause
fibrous motions, it is termed volition; which is said in Sect. V. to be a
motion of the central parts, or of the whole sensorium, _terminating_ in