can be given empirically, although this space itself is not empirically given, and is no object of experience—a paradox deserving to be solved.
Proposition 3.
In every motion of a body, whereby it is moving in respect of another, an opposite and equal motion of the latter is necessary.
Demonstration.
According to the third law of mechanics (Proposition 4) the communication of the motion of a body is only possible through the community of its original moving forces, and these only through reciprocal and equal motion. The motion of both is then real. But as the reality of this motion does not rest (as in the second proposition) on the influence of external forces, but follows immediately and inevitably from the conception of the relation of the moved in space, to every other [thing] thereby movable, the motion of the latter is necessary.
Observation.
This proposition determines the modality of motion in respect of mechanics; that, for the rest, these three propositions determine the motion of matter in respect of its possibility, reality, and necessity, in short, in respect of all the three categories of Modality, is sufficiently obvious of itself.
General Observation on Phenomenology.
There are thus three conceptions noticeable here, whose employment in universal natural science is unavoidable, and whose exact definition is for this reason necessary, although not so easy and comprehensible: firstly, the conception of motion in relative (movable) space; secondly, the conception of motion in absolute (immovable) space; thirdly, the conception of relative motion generally, as distinguished from absolute [motion.] The conception of
