only know in part how to reduce
physiology to chemistry, but we know enough to make it likely that the
reduction is possible. If we suppose it effected, what would become of
the difference between vital and mechanical movements?

Some analogies will make the difference clear. A shock to a mass of
dynamite produces quite different effects from an equal shock to a mass
of steel: in the one case there is a vast explosion, while in the other
case there is hardly any noticeable disturbance. Similarly, you may
sometimes find on a mountain-side a large rock poised so delicately that
a touch will set it crashing down into the valley, while the rocks all
round are so firm that only a considerable force can dislodge them What
is analogous in these two cases is the existence of a great store of
energy in unstable equilibrium ready to burst into violent motion by
the addition of a very slight disturbance. Similarly, it requires only
a very slight expenditure of energy to send a post-card with the words
"All is discovered; fly!" but the effect in generating kinetic energy
is said to be amazing. A human body, like a mass of dynamite, contains
a store of energy in unstable equilibrium, ready to be directed in this
direction or that by a disturbance which is physically very small,
such as a spoken word. In all such cases the reduction of behaviour to
physical laws can only be effected by entering into great minuteness; so
long as we confine ourselves to the observation of comparatively
large masses, the way in which the equilibrium will be upset cannot be
determined. Physicists distinguish between macroscopic and microscopic
equations: the former determine the visible movements of bodies of
ordinary size, the latter the minute occurrences in the smallest parts.
It is only the microscopic equations that are supposed to be the same
for all sorts of matter. The macroscopic equations result from a process
of averaging out, and may be different in different cases. So, in our
instance, the laws of macroscopic phenomena are different for mechanical
and vital movements, though the laws of microscopic phenomena may be the
same.

We may say, speaking somewhat roughly, that a stimulus applied to the
nervous system, like a spark to dynamite, is able to take advantage of
the stored energy in unstable equilibrium, and thus to produce movements
out of proportion to the proximate cause. Movements produced in this way
are vital movements, while mecha