Newton's first law of motion states that, under the action of no forces, a body continues at rest or in uniform rectilinear motion. This statement, as it stands, is meaningless, if we do not assume the Absolute Theory, and is a mere pious opinion incapable of verification or refutation if we do assume that theory. If we assume the Relational Theory, it is an incomplete statement. A uniform motion is one that covers equal distances during the same number of repetitions of some standard process which is itself isochronous. The material systems which affect the motions of bodies do also produce some measurable effects, such as balancing weights on strings over pulleys, or stretching spring-balances. The factor of mass actually enters into the traditional Mechanics in two quite different ways. The first is the mass that is mentioned in the law of gravitation; the second is the mass which is involved in the second law of motion.