same point of the surface, but moves itself from east to west, I shall still infer the foregoing motion of the earth on its axis, and both perceptions are a sufficient proof of the reality of this motion, for which the change of relation to external space (the starry heaven) is inadequate as it is mere phenomenon, which may proceed from two actually opposed causes, and which is not a cognition deducible from the ground of explanation of all phenomena of this change, that is, experience. But that this motion, although no change of relation to empirical space, is nevertheless no absolute motion, but continuous change of the relation of matters to one another, and while conceived in absolute space, is really only relative and for this very reason, alone true motion; this rests on the conception of the reciprocally continuous retreat of each part of the earth (outside the axis) from every other [part], situated opposite to it in the diameter, at equal distance from the centre. For this motion is real in absolute space, in that thereby the retreat from the distance in question, when gravity in itself would attract to the body, and indeed without any dynamical repulsive cause (as may be seen from the instances chosen in Newton's Princ. Phil. Nat., p. 10, Edit. 1711),[1] is continuously replaced by real motion inclosed within the moved matter (namely, the centre of the same), but not having reference to the external space.
As to the case of the third proposition, it does not require, in order to show the truth of the reciprocally opposed and equal motion of two bodies even without reference to the empirical space, [to exhibit] the active dynamical influence (of gravity or of a distended string) given through experience, which is necessary in the second case, but the mere dynamical possibility of such an influence
- ↑ He there says: Motus quidem veros corporum singulorum cognoscere et ab apparentibus actu discriminare difficillimum est; propterea quod partis spatii illius immobilis, in quo corpora vere moventur, rion incurrunt in sensus. Causa tamen non est prorsa disparata. Thereupon he allows two spheres attached by a thread, to turn about their common centre of gravity in empty space, and shows how the reality of their motion, together with its direction, can nevertheless be found in experience. I have also sought to demonstrate this under somewhat altered circumstances from the earth as moved on its axis.
