3. As the sensations of pleasure and pain are originally introduced by the
irritations of external objects: so our desires and aversions are
originally introduced by those sensations; for when the objects of our
pleasures or pains are at a distance, and we cannot instantaneously possess
the one, or avoid the other, then desire or aversion is produced, and a
voluntary exertion of our ideas or muscles succeeds.

The pain of hunger excites you to look out for food, the tree, that shades
you, presents its odoriferous fruit before your eyes, you approach, pluck,
and eat.

The various movements of walking to the tree, gathering the fruit, and
masticating it, are associated motions introduced by their connection with
sensation; but if from the uncommon height of the tree, the fruit be
inaccessible, and you are prevented from quickly possessing the intended
pleasure, desire is produced. The consequence of this desire is, first, a
deliberation about the means to gain the object of pleasure in process of
time, as it cannot be procured immediately; and, secondly, the muscular
action necessary for this purpose.

You voluntarily call up all your ideas of causation, that are related to
the effect you desire, and voluntarily examine and compare them, and at
length determine whether to ascend the tree, or to gather stones from the
neighbouring brook, is easier to practise, or more promising of success;
and, finally, you gather the stones, and repeatedly fling them to dislodge
the fruit.

Hence then we gain a criterion to distinguish voluntary acts or thoughts
from those caused by sensation. As the former are always employed about the
_means_ to acquire pleasurable objects, or the _means_ to avoid painful