has a real cause, that it emanates from a force. But we must understand what we mean by this last word. In natural science force is only a function of mass and velocity: it is measured by acceleration: it is known and estimated only by the movements which it is supposed to produce in space. One with these movements, it shares their relativity. Hence the physicists, who seek the principle of absolute motion in force so defined, are led by the logic of their system back to the hypothesis of an absolute space which they had at first desired to avoid.[1] So it will become necessary to take refuge in the metaphysical sense of the word, and attribute the motion which we perceive in space to profound causes, analogous to those which our consciousness believes it discovers within the feeling of effort. But is the feeling of effort really the sense of a profound cause? Have not decisive analyses shown that there is nothing in this feeling other than the consciousness of movements already effected or begun at the periphery of the body? It is in vain, then, that we seek to found the reality of motion on a cause which is distinct from it: analysis always brings us back to motion itself.
But why seek elsewhere? So long as we apply a movement to the line along which it passes, the same point will appear to us, by turns, according to the points or the axes to which we
- ↑ Newton, in particular.
