I have pointed out that the growth of clear and definite views respecting the constitution of matter has led to the conclusion that, so far as natural agencies are concerned, it is ingenerable and indestructible.  In so far as matter may be conceived to exist in a purely passive state, it is, imaginably, older than motion.  But, as it must be assumed to be susceptible of motion, a particle of bare matter 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 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