at rest must be endowed with the potentiality of motion. Such a
particle, however, by the supposition, can have no energy, for there
is no cause why it should move. Suppose now that it receives an
impulse, it will begin to move with a velocity inversely proportional
to its mass, on the one hand, and directly proportional to the
strength of the impulse, on the other, and will possess _kinetic
energy_, in virtue of which it will not only continue to move for ever
if unimpeded, but if it impinges on another such particle, it will
impart more or less of its motion, to the latter. Let it be conceived
that the particle acquires a tendency to move, and that nevertheless
it does not move. It is then in a condition totally different from
that in which it was at first. A cause competent to produce motion is
operating upon it, but, for some reason or other, is unable to give
rise to motion. If the obstacle is removed, the energy which was
there, but could not manifest itself, at once gives rise to motion.
While the restraint lasts, the energy of the particle is merely
potential; and the case supposed illustrates what is meant by
_potential energy_. In this contrast of the potential with the actual,
modern physics is turning to account the most familiar of Aristotelian
distinctions--that between dunamis and energeia.

That kinetic energy appears to be imparted by impact is a fact of
daily and hourly experience: we see bodies set in motion by bodies,
already in motion, which seem to come in contact with them. It is a
truth which could have been learned by nothing but experience, and
which cannot be explained, but must be taken as an ultimate fact
about which, explicable or inexplicable, there can be no doubt.
Strictly speaking, we have no direct apprehension of any other cause
of motion. But experience furnishes innumerable examples of the
production of kinetic energy in a body previously at rest, when no