I was thinking through a thought experiment which led me to conclude that mass is relative. My thought process was basically this; there are two objects moving past each other. In the reference frame of either object, they are at rest and the other object is moving. Since the other object is moving, it has a higher kinetic energy than the reference object. This means that its mass, or the ratio of force to acceleration, would be higher for the 'moving' object than for the 'observer' object. Now, if the inertial mass is relative, is gravitational mass also relative? And if now, does that mean that we can find the 'absolute mass' of a particle or body by measuring the gravitational force it exerts?