, relative to which a system of axes is defined, and with reference to which the law of inertia is to hold (Inertial system, vide [Note 15]). The alternatives with which one is faced are highly unsatisfactory. The introduction of absolute space gives rise to the oft-discussed conceptual difficulties which have gnawed at the foundations of Newton's mechanics. The introduction of the system of reference
certainly takes the relativity of motions so far into account, that all systems in uniform motion relative to an
-system are established as equivalent from the very outset, but we can affirm with certainty that there is no such thing as a visible
-system, and that we shall never succeed in arriving at a final determination of such a system. (It will, at most, be possible, by progressively taking account of the influences of constellations upon the solar system and upon one another, to approximate to a system of co-ordinates, which could play the part of such an inertial system with a sufficient degree of accuracy.) As a result of this objection, the founder of the view himself, C. Neumann, admits that it will always be somewhat unsatisfactory and enigmatical, and that mechanics, based on this principle, would indeed be a very peculiar theory.
It therefore seems quite natural that E. Mach (vide [Note 16]) should be led to propose that the law of inertia be so formulated that its relations to the stellar bodies are directly apparent. "Instead of saying that the direction and speed of a mass
