"In a system in which only two particles or masses, withdrawn from outer influences, are determined to motion by mutual attraction and the influence of a primary impulse in another direction, calculation shows us that, motion to infinity being excluded, the attainment, and indeed the immediate attainment, of full stability is a necessity; and for swinging pendula and vibrating strings it may be calculated, from the nature of the moving forces, that they would remain in a condition of fully stable motion if outer resistance were removed; for, such obstacles present, they pass through an approximately stable condition to one of absolute stability. The power of purely mathematical calculation does not go beyond such comparatively simple cases.... "But if we call experience to our aid, it may be asserted, in accordance with very general facts, that, in a system left to itself or under constant outer conditions, and starting from any conceivable state, if not full stability at least a greater or less approximation to it is reached as final condition, from which no retrogression takes place through the inner workings of the system itself. The tendency to approximately stable conditions appears, or the actual state is attained, according to the measure in which variable outer influences are withdrawn. So that so little is lacking to our hypothesis, that, although it has at this point to make up for the impossibility of perfect demonstration, we are nevertheless justified in laying down the following law or principle:-- "In every system of material parts left to itself or under constant outer influences, so, then, in the material system of the universe, in so far as we regard it as isolated, there takes place, motion to infinity being excluded, a continuous progress from more unstable to more stable conditions, up to the attainment of a final condition of full or approximate stability." From the union of the principle thus stated and that of the conservation of energy "it follows that no unlimited progress of the universe to absolute stability, which consists in perfect rest of the parts, can take place.... The energy manifested in the universe cannot be altered, in general, in its amount, but only in the form in which it manifests itself." "It cannot be asserted that the attainment of full stability in the universe would be the attainment of an eternal rest, but only of the most perfectly adjusted motions, and therefore such motions as would give rise to no variations.... But a condition which brings with it eternal repetition cannot be reached in finite time."