 But, uniquely, Newton immediately went on to consider the deeper theoretical significance of these principles: they radically reconceptualize the problem of “true motion” in the planetary system. First, they implied that the entire system must be seen as included in a space that may, itself, be either at rest or in uniform motion. Second, they implied that the only truly fixed point in such a system is the center of gravity of the relevant bodies. This, too, may therefore be in uniform motion or at rest: