Classical physics appears to just accept the fact that inertial mass and gravitation mass are the same: they just turn up as the same in the equations with G and g but without reason. Experiment shows they are the same to a very high accuracy. However, there is no explanation about why that which 'causes' inertia and that which 'causes' weight should be the same. This may well have led to the confusion between mass and weight; many people think you can push a roller skate or a railway truck with equal ease where there is no gravity. Einstein's Principle of Equivalence uses the equality of inertial mass and gravitational mass to develop the General Theory of Relativity.