 Oddly enough, no one in the 17th century, or even before the late 19th century, expressed this equivalence of reference-frames any more clearly than Newton himself. However, the credit for articulating this equivalence precisely for the first time, belongs to Christiaan Huygens, who introduced it as one of the Hypotheses of his first work on the rules of impact (1656). “Hypothesis I” was the first clear statement of the principle of inertia: “Any body, once in motion, if nothing opposes it, continues to move always with the same velocity and along the same straight line” (1656, pp. 30–31). The first precise statement of the relativity principle followed as Hypothesis III: 