 So in this regard absolute velocity is quite unlike acceleration (including rotation). Newtonian acceleration is understood in absolute space as the rate of change of absolute velocity, and is, according to Newtonian mechanics, generally measurable; for instance by measuring the height that the water ascends the sides of the  bucket.[6]  Leibniz argued (rather inconsistently, as we shall see) that since differences in absolute velocity are unobservable, they are not be genuine differences at all; and hence that Newton’s absolute space, whose existence would entail the reality of such differences, must also be a fiction. Few philosophers today would immediately reject a quantity as unreal simply because it was not experimentally determinable, but this fact does justify genuine doubts about the reality of absolute velocity, and hence of absolute space.