e course of the animal spirits, which consist
of matter in motion. Thus the soul becomes a centre of force. But, at
the same time, the distinction between spirit and matter vanishes;
inasmuch as matter, according to a tenable hypothesis, may be nothing
but a multitude of centres of force. The case is worse if we adopt the
modern vague notion that consciousness is seated in the grey matter of
the cerebrum, generally; for, as the grey matter has extension, that
which is lodged in it must also have extension. And thus we are led, in
another way, to lose spirit in matter.

In truth, Descartes' physiology, like the modern physiology of which it
anticipates the spirit, leads straight to Materialism, so far as that
title is rightly applicable to the doctrine that we have no knowledge
of any thinking substance, apart from extended substance; and that
thought is as much a function of matter as motion is. Thus we arrive at
the singular result that, of the two paths opened up to us in the
"Discourse upon Method," the one leads, by way of Berkeley and Hume, to
Kant and Idealism; while the other leads, by way of De La Mettrie and
Priestley, to modern physiology and Materialism.[76] Our stem divides
into two main branches, which grow in opposite ways, and bear flowers
which look as different as they can well be. But each branch is sound
and healthy, and has as much life and vigour as the other.

If a botanist found this state of things in a new plant, I imagine that
he might be inclined to think that his tree was monoecious--that the
flowers were of different sexes, and that, so far from setting up a
barrier between the two branches of the tree, the only hope of fertility
lay in bringing them together. I may be taking too much of a
naturalist's view of the case, but I must confess that this is exactly
my notion of what is to be done with metaphysics and physics. Their
differences are complementary, not antagonistic; and thought will never
be completely fruitful until the one unites with the other. Let me try
to explain what I mean. I hold, with the Materialist, that the human
body, like all living bodies, is a machine, all the operations of which
will, sooner or later, be explained on physical principles. I believe
that we shall, sooner or later, arrive at a mechanical equivalent of
consciousness, just as we have arrived at a mechanical equivalent of
heat. If a pound weight falling through a distance of a foot gives rise
to a defini