*THE POPULAR SCIENCE MONTHLY.*

themselves from the prejudice that nothing can be real which is not absolute.

Prof. Neumann is not content with showing, or attempting to show, that the reality of motion necessitates its reference to a rigid body unchangeable in its position in space, but he seeks to verify this assumption by asking himself the question what consequences would ensue, on the hypothesis of the mere relativity of motion, if all bodies in space, except one, were annihilated. "Let us suppose," he says (*loc. cit.*, p. 27), "that among the stars there is one which consists of fluid matter, and which, like our earth, is in rotary motion around an axis passing through its centre. In consequence of this motion, by virtue of the centrifugal forces developed by it, this star will have the form of an ellipsoid. What form, now, I ask, will this star assume if suddenly all other celestial bodies are annihilated?

"These centrifugal forces depend solely upon the state of the star itself; they are wholly independent of the other celestial bodies. These forces, therefore, as well as the ellipsoidal form, will persist, irrespective of the continued existence or disappearance of the other bodies. But, if motion is defined as something relative—as a relative change of place of two points—the answer is very different. If, on this assumption, we suppose all other celestial bodies to be annihilated, nothing remains but the material points of which the star in question itself consists. But, then, these points do not change their relative positions, and are therefore at rest. It follows that the star must be at rest at the moment when the annihilation of the other bodies takes place, and therefore must assume the spherical form taken by all bodies in a state of rest. A contradiction so intolerable can be avoided only by abandoning the assumption of the relativity of motion, and conceiving motion as absolute, so that thus we are again led to the principle of the body Alpha."

This reasoning of Prof. Neumann is irrefutable, if we concede the admissibility of his hypothesis of the destruction of all bodies in space but one. But the very principle of relativity forbids such an hypothesis. The annihilation of all bodies but one would not only destroy the *motion* of this one remaining body and bring it to rest, as Prof. Neumann sees, but it would also destroy its very *existence* and bring it to naught, as he does not see. A body cannot survive the system of relations in which alone it has its being; its *presence or position* in space is no more possible without reference to other bodies than its *change of position or presence* is possible without such reference; and, as I have abundantly shown, all properties of a body are in their nature relations, and imply terms beyond the body itself. The case put by Prof. Neumann is thus an attestation of the truth that the essential relativity of all physical reality implies the persistence both of force and of matter, so that his argument is a demonstration, not of the falsity, but of the truth of the principle of relativity.