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 which are, in a measure, logical elements, to which may be joined a third element, itself experimental, having its foundations in our sensations—namely, force, work, or power.

The ideas of force, work, and power, are drawn from the experience man has of his muscular activity. Nevertheless the greatest mathematical minds from from Descartes to Leibniz have been obliged to define and explain them clearly.

Force.—The prototype of force is weight, universal attraction. Experiment shows us that every body falls as long as no obstacle opposes its fall. This is so universal a property of matter that it serves to define it. The force, weight, is therefore the name given to the cause of the fall of the bodies.

Force in general is the cause of motion. Hence force exists only in so far as there is motion. There would be no force without action. This is Newton's point of view. It did not prevail, and was not the point of view of his successors. The name of force has been given not only to the cause which produces or modifies motion, but to the cause which resists and prevents it. And then not only have forces in action been considered (dynamics), but forces at rest (statics). Now, to Newton there was no statics. Forces do not continue to exist when they produce no motion; they are not in equilibrium, they are destroyed.

The idea of force therefore is a metaphysical idea which contains the idea of cause. But it becomes experimental immediately it is looked upon as resisting motion, according to the point of view of Newton's opponents. Its foundations lie in the muscular activity of man.

Man can support a burden without bending or