 Einstein began to see this complete relativization as possible in 1907, thanks to his discovery of the Equivalence Principle (cf. Lehmkuhl forthcoming). Imagine we are far out in space, in a rocket ship accelerating at a constant rate  \(g = 9.81 m/s^2.\) Things will feel just like they do on the surface of the Earth; we will feel a clear up-down direction, bodies will fall to the floor when released, etc. Indeed, due to the well-known empirical fact that gravity affects all bodies by imparting a force proportional to their matter (and energy) content, independent of their internal constitution, we know that any experiment performed on this rocket will give the same results that the same experiment would give if performed on the Earth. Now, Newtonian theory teaches us to consider the apparent downward, gravity-like forces in the rocket ship as ‘pseudo-forces’ or ‘inertial forces’, and insists that they are to be explained by the fact that the ship is accelerating in absolute space. But Einstein asked whether there is any way for the person in the rocket to regard him/herself as being ‘at rest’ rather than in absolute (accelerated) motion? And the answer he gave is: Yes. The rocket traveler may regard him/herself as being ‘at rest’ in a homogeneous and uniform gravitational field. Such a field would entail an accelerative force “downward” on every body that is equal in magnitude and direction everywhere in space. This is unlike the Earth’s gravitational field, which varies depending on distance from the Earth’s center and points in different directions at different locations. Positing the existence of such a field will explain all the observational facts just as well as the supposition that he/she is accelerating relative to absolute space (or, absolutely accelerating in Minkowski spacetime). But is it not clear that the latter is the truth, while the former is a fiction? By no means; if there were a uniform gravitational field filling all space, then it would affect all the other bodies in the world – the Earth, the stars, etc, – imparting to them a downward acceleration away from the rocket; and that is exactly what the traveler observes.