the immediate cause of volition has generally been termed _a motive_; and
that of irritation only has generally obtained the name of _stimulus_; yet
as the immediate cause, which excites the sensorial powers of sensation, or
of association into exertion, have obtained no general name, we shall use
the word stimulus for them all.

Hence the quantity of motion produced in any particular part of the animal
system will be as the quantity of stimulus and the quantity of sensorial
power, or spirit of animation, residing in the contracting fibres. Where
both these quantities are great, _strength_ is produced, when that word is
applied to the motions of animal bodies. Where either of them is deficient,
_weakness_ is produced, as applied to the motions of animal bodies.

Now as the sensorial power, or spirit of animation, is perpetually
exhausted by the expenditure of it in fibrous contractions, and is
perpetually renewed by the secretion or production of it in the brain and
spinal marrow, the quantity of animal strength must be in a perpetual state
of fluctuation on this account; and if to this be added the unceasing
variation of all the four kinds of stimulus above described, which produce
the exertions of the sensorial powers, the ceaseless vicissitude of animal
strength becomes easily comprehended.

If the quantity of sensorial power remains the same, and the quantity of
stimulus be lessened, a weakness of the fibrous contractions ensues, which
may be denominated _debility from defect of stimulus_. If the quantity of
stimulus remains the same, and the quantity of sensorial power be lessened,
another kind of weakness ensues, which may be termed _debility from defect
of sensorial power_; the former of these is called by Dr. Brown, in his
Elements of Medicine, direct debility, and the latter indirect debility.
The coincidence of some parts of this work with correspondent deductions in