ones; while the latter are employed in the possession of those, which are
already in our power.

Hence the activity of this power of volition produces the great difference
between the human and the brute creation. The ideas and the actions of
brutes are almost perpetually employed about their present pleasures, or
their present pains; and, except in the few instances which are mentioned
in Section XVI, on instinct, they seldom busy themselves about the means of
procuring future bliss, or of avoiding future misery; so that the acquiring
of languages, the making of tools, and labouring for money, which are all
only the means to procure pleasures; and the praying to the Deity, as
another means to procure happiness, are characteristic of human nature.

4. As there are many diseases produced by the quantity of the sensation of
pain or pleasure being too great or too little; so are there diseases
produced by the susceptibility of the constitution to motions causable by
these sensations being too dull or too vivid. This susceptibility of the
system to sensitive motions is termed sensibility, to distinguish it from
sensation, which is the actual existence or exertion of pain or pleasure.

Other classes of diseases are owing to the excessive promptitude, or
sluggishness of the constitution to voluntary exertions, as well as to the
quantity of desire or of aversion. This susceptibility of the system to
voluntary motions is termed voluntarity, to distinguish it from volition,
which is the exertion of desire or aversion; these diseases will be treated
of at length in the progress of the work.

_Association._

III. 1. It is not easy to assign a cause, why those animal movements, that