t yet containing within itself the quantifiable
capacity of transformation. The objective correlatives of the different
classes of sensible experiences are found to be different forms which
this Energy assumes--the kinetic energy of a mass in motion, the radiant
energy of Light, the energy of Heat, the potential energy of chemical
separation, etc.--all these have now at length been shown to be forms of
one real thing capable under appropriate conditions of being transmuted
into each other and of which not only the inter-transmutability but the
equivalent values can be calculated and have been found by experiment to
be fixed and definite. Thus the mechanical equivalent of heat is a fixed
and definite quantity. The Energy of a body in motion can be measured
and stated in terms of mass and velocity.

The profound conception of Aristotle, under which Nature was regarded as
a potent Energy containing within itself the capacity of generating the
phenomenal World, has again been revived and realised--but with great
additions. The theory in the hands of Science is now not only confirmed
by incessant experiment, but the relation which it affirms between
reality and phenomenon has been _quantified_.

Moreover, the actual operations under which the potential generates the
actual have, so to say, been laid bare to view; and lastly, the
inter-transmutability of all forms of Energy and its real unity have
been established.

The doctrine has therefore received a confirmation of which Aristotle
did not dream, and its explanation has at the same time received an
illumination which his vague if profound adumbration could never afford.
With this added support the true conception of human knowledge has
received new strength. The theory is still, nevertheless, not to be
grasped without a resolute effort of reflection. It involves an
inversion of our everyday conceptions more radical than that which was
demanded by the Copernican theory of astronomy, and we know that that
theory--offered to and rejected by mankind before the beginning of the
Christian era--had to wait through sixteen or seventeen hundred years
before it secured an acceptance, at first grudging and even now not
always adequate.

* * * * *

The ordinary metaphysical student has hitherto rather resented the idea
that in order to a true solution of the problem of Knowledge he must
acquaint himself with the fundamental conceptions of physics. Y