All that I have said of ‹materials› is equally applicable to the ‹forces› which Nature places at our disposal. Gravitation, the elasticity of air, the power of the winds, the laws of equilibrium, vegetable life, animal life, are so many forces which we learn to turn to account. The pains and intelligence which we bestow in this way always admit of remuneration, for we are not bound to devote our efforts to the advantage of others gratuitously. But these natural forces, in themselves, and apart from all intellectual or bodily exertion, are ‹gratuitous› gifts of Providence, and in this respect they remain destitute of ‹value› through all the complications of human transactions. This is the leading idea of the present work. This observation would be of little importance, I allow, if the co-operation of Nature were constantly uniform, if each man, at all times, in all places, in all circumstances, received from Nature equal and invariable assistance. In that case, science would be justified in not taking into account an element which, remaining always and everywhere the same, would affect the services [p090] exchanged in equal proportions on both sides. As in geometry we eliminate portions of lines common to two figures which we compare with each other, we might neglect a co-operation which is invariably present, and content ourselves with saying, as we have done hitherto, “There is such a thing as natural wealth—Political Economy acknowledges it, and has no more concern with it.”