According to Newton, motion takes place everywhere in a uniform and immovable, absolute space. Whether a body is in motion relative to this space can be concluded from the causes and effects of the movement.
If a bucket filled with water is made to rotate, the water pushes outwards because of its inertia and rises up the sides of the bucket the more it is set itself into rotation by the rotating bucket. The rising of the water reveals that the bucket is in motion relative to the absolute space.