_Thirdly,_ We learn from anatomy, that the immediate object of power in
voluntary motion, is not the member itself which is moved, but certain
muscles, and nerves, and animal spirits, and, perhaps, something still
more minute and more unknown, through which the motion is successively
propagated, ere it reach the member itself whose motion is the immediate
object of volition. Can there be a more certain proof, that the power,
by which this whole operation is performed, so far from being directly
and fully known by an inward sentiment or consciousness, is, to the last
degree mysterious and unintelligible? Here the mind wills a certain
event: Immediately another event, unknown to ourselves, and totally
different from the one intended, is produced: This event produces
another, equally unknown: Till at last, through a long succession, the
desired event is produced. But if the original power were felt, it must
be known: Were it known, its effect also must be known; since all power
is relative to its effect. And _vice versa,_ if the effect be not known,
the power cannot be known nor felt. How indeed can we be conscious of a
power to move our limbs, when we have no such power; but only that to
move certain animal spirits, which, though they produce at last the
motion of our limbs, yet operate in such a manner as is wholly beyond
our comprehension?

We may, therefore, conclude from the whole, I hope, without any
temerity, though with assurance; that our idea of power is not copied
from any sentiment or consciousness of power within ourselves, when we
give rise to animal motion, or apply our limbs to their proper use and
office. That their motion follows the command of the will is a matter of
common experience, like other natural events: But the power or energy by
which this is effected, like that in other natural events, is unknown
and inconceivable.[12]