to be, from its extreme minuteness, 'invisible'
and 'insensible,' or, as we should now say, molecular, is likewise
evident. I can therefore imagine the delight with which he would hear me
speak when I say, that it is no longer a matter of keen-sighted
speculation, but a matter of carefully demonstrated fact, that all our
knowledge of the external world is nothing more than a knowledge of
motion. For all the forms of energy have now been proved to be but modes
of motion; and even matter, if not in its ultimate constitution vortical
motion, at all events is known to us only as changes of motion: all that
we perceive in what we call matter is change in modes of motion. We do
not even know what it is that moves; we only know that when some modes
of motion pass into other modes, we perceive what we understand by
matter. It would take me too long to justify this general statement so
that it should be intelligible to every one; but I am confident that all
persons who understand such subjects will, when they think about it,
accept this general statement as one which is universally true. And, if
so, they will agree with Hobbes that all our knowledge of the external
world is a knowledge of motion.

Now, if it would have been thus a joy to Hobbes to have heard to-day how
thoroughly he has been justified in his views touching the external
world, with no less joy would he have heard that he has been equally
justified in his views touching the internal world. For it has now been
proved, beyond the possibility of dispute, that it is only in virtue of
those invisible movements which he inferred that the nervous system is
enabled to perform its varied functions.

To many among the different kinds of movement going on in the external
world, the animal body is adapted to respond by its own movements as
best suits its own welfare; and the mechanism whereby this is effected
is the neuro-muscular system. Those kinds of movement going on in the
external world which are competent to evoke responsive movements in the
animal body are called by physiologists stimuli. When a stimulus falls
upon the appropriate sensory surface, a wave of molecular movement is
sent up the attached sensory nerve to a nerve-centre, which thereupon
issues another wave of molecular movement down a motor nerve to the
group of muscles over whose action it presides; and when the muscles
receive this wave of nervous influence they contract. This kind of
response to stimul