  The contention that the empirical reasoning in the Principia does not presuppose an unbridled form of absolute time and space should not be taken as suggesting that Newton's theory is free of fundamental assumptions about time and space that have subsequently proved to be problematic. For example, in the case of space, Newton presupposes that the geometric structure governing which lines are parallel and what the distances are between two points is three-dimensional and Euclidean. In the case of time Newton presupposes that, with suitable corrections for such factors as the speed of light, questions about whether two celestial events happened at the same time can in principle always have a definite answer. And the appeal to forces to distinguish real from apparent non-inertial motions presupposes that free-fall under gravity can always, at least in principle, be distinguished from inertial  motion.[20]