for the goad of a spur on the skin of a horse shall induce him to move a
load of hay. They differ from the gravitating motions there mentioned as
they are exerted with equal facility in all directions, and they differ
from the chemical class of motions, because no apparent decompositions or
new combinations are produced in the moving materials.

Hence, when we say animal motion is excited by irritation, we do not mean
that the motion bears any proportion to the mechanical impulse of the
stimulus; nor that it is affected by the general gravitation of the two
bodies; nor by their chemical properties, but solely that certain animal
fibres are excited into action by something external to the moving organ.

In this sense the stimulus of the blood produces the contractions of the
heart; and the substances we take into our stomach and bowels stimulate
them to perform their necessary functions. The rays of light excite the
retina into animal motion by their stimulus; at the same time that those
rays of light themselves are physically converged to a focus by the
inactive humours of the eye. The vibrations of the air stimulate the
auditory nerve into animal action; while it is probable that the tympanum
of the ear at the same time undergoes a mechanical vibration.

To render this circumstance more easy to be comprehended, _motion may be
defined to be a variation of figure_; for the whole universe may be
considered as one thing possessing a certain figure; the motions of any of
its parts are a variation of this figure of the whole: this definition of
motion will be further explained in Section XIV. 2. 2. on the production of
ideas.

Now the motions of an organ of sense are a succession of configurations of
that organ; these configurations succeed each other quicker or slower; and