 Similarly, we might ask if hitting one of the balls with a hammer exactly as they collide will result in changes to the balls’ motion after they collide or before they collide. Intuitively it seems that the force exerted by the hammer will causally influence the ball’s subsequent motion. But it is not obvious how this verdict is borne out purely by considering Newton’s equations: singling out the forward-looking counterfactual as the correct one seems arbitrary. As John Earman argues,