In a certain sense, this is just an extension of relativity, albeit a radical one. Aristotle was first to emphasize that we only perceive relative speed. On a ship, for example, we talk of our speed relative to the ship; on land, relative to Earth. Galileo understood that this is the reason why the Earth can move with respect to the sun without us feeling the movement. Speed is not a property of an object on its own: it is the property of the motion of an object with respect to another object.
