The equivalence principle states that an accelerating observer who has no external information (view of fixed stars, etc) can in principle not perform an experiment to determine whether he is either undergoing linear acceleration or at rest in a gravitational field. This leads to the equivalence of inertial and gravitational mass. Is there an underlying reason this should, or could, be true? For instance (feel free to offer corrections...) the higgs field only couples to certain particles involved in the weak nuclear force, and in this case only causes them to have an inertial mass. The equivalence of inertial and gravitational mass is just purely a postulate. I know this is one of those statements that appears obvious, but what is the deeper reasoning behind it?