"A little reflection will show that law of the inert and the gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton's equation of motion in a gravitational field, written out in full, is (Inert mass) . (Acceleration) = (Intensity of the gravitational field) . (Gravitational mass). It is only when there is numerical equality between the inert and gravitational mass that the acceleration is independent of the nature of the body."