d 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
impact is discernible as the cause of that energy. In all such cases,
the presence of a second body is a necessary condition; and the amount
of kinetic energy, which its presence enables the first to gain, is
strictly dependent on the relative positions of the two. Hence the
phrase _energy of position_, which is frequently used as equivalent to
potential energy. If a stone is picked up and held, say, six feet
above the ground, it has _potential energy_, because, if let go, it
will immediately begin to move towards the earth; and this energy may
be said to be _energy of position_, because it depends upon the
relative position of the earth and the stone. The stone is solicited
to move but cannot, so long as the muscular strength of the holder
prevents the solicitation from taking effect. The stone, therefore,
has potential energy, which becomes kinetic if it is let go, and the
amount of that kinetic energy which will be developed before it
strikes the earth depends on its position--on the fact that it is,
say, six feet off the earth, neither more nor less. Moreover, it can
be proved that the raiser of the stone had to exert as much energy in
order to place it in its position, as it will develop in falling.
Hence the energy which was exerted, and apparently exhausted, in
raising the stone, is potentially in the stone, in its raised
position, and will manifest itself when the stone is set free. Thus
the energy, withdrawn from the general stock to raise the stone, is
return