culty, and occasionally produces contraction of the
fibres; this pleasure or pain is therefore to be considered as another
stimulus, which may either act alone or in conjunction with the former
faculty of the sensorium termed irritation.

This new stimulus of pleasure or pain either induces into action the
sensorial faculty termed sensation, which then produces the contraction of
the fibres; or it introduces desire or aversion, which excite into action
another sensorial faculty, termed volition, and may therefore be considered
as another stimulus, which either alone or in conjunction with one or both
of the two former faculties of the sensorium produces the contraction of
animal fibres. There is another sensorial power, that of association, which
perpetually, in conjunction with one or more of the above, and frequently
singly, produces the contraction of animal fibres, and which is itself
excited into action by the previous motions of contracting fibres.

Now as the sensorial power, termed irritation, residing in any particular
fibres, is excited into exertion by the stimulus of external bodies acting
on those fibres; the sensorial power, termed sensation, residing in any
particular fibres is excited into exertion by the stimulus of pleasure or
pain acting on those fibres; the sensorial power, termed volition, residing
in any particular fibres is excited into exertion by the stimulus of desire
or aversion; and the sensorial power, termed association, residing in any
particular fibres, is excited into action by the stimulus of other fibrous
motions, which had frequently preceded them. The word stimulus may
therefore be used without impropriety of language, for any of these four
causes, which excite the four sensorial powers into exertion. For though
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