 Newton’s proposal for understanding motion solves the problems that he posed for Descartes, and provides an interpretation of the concepts of constant motion and acceleration that appear in his laws of motion. However, it suffers from two notable interpretational problems, both of which were pressed forcefully by Leibniz (in the Leibniz-Clarke Correspondence, 1715–1716) – which is not to say that Leibniz himself offered a superior account of motion (see  below).[5]  First, according to this account, absolute velocity is a well-defined quantity: more simply, the absolute speed of a body is the rate of change of its position relative to an arbitrary point of absolute space. But the Galilean relativity of Newton’s laws (see the entry on  space and time: inertial frames)  means that the evolution of a closed system would have been identical if it had been moving at a different (constant) overall velocity: as Galileo noted in his (see the entry on  Galileo Galilei),  an experimenter cannot determine from observations inside his cabin whether his ship is at rest in harbor or sailing smoothly. Put another way, according to Newtonian mechanics, in principle Newton’s absolute velocity cannot be experimentally determined.