Modern physics recognizes three varieties of mass.

*Inertial* mass is the *m* in *F = ma*. That is, it describes how difficult it is to change an object's velocity.

*Passive gravitational* mass is the *m* in *W = mg*. It describes the force on an object in a gravitational field of a given strength.

*Active gravitational* mass is the *M* in *-GM/r*. It describes the strength of the gravitational field emanated by an object.

Newtonian gravitation *assumes without proof* that these three masses are the same for every object in the universe. General relativity *redefines gravity* so that these three masses are tautologically equivalent.

However, there seems to be no *naturally essential* reason that these masses should be equal. In other words, there is no well established theory of physics that does not directly define these quantities but that you can use to prove that they are equal, unless that theory simply redefines gravity with the intention of ensuring that the quantities will be equal.

Given that, it seems reasonable for FE theory to be just as arbitrary: i.e. to assert that *some* objects in the universe have active gravitational mass and *some* have passive gravitational mass, and that while those sets overlap they certainly are not identical, nor do they include the entire universe.