The Equivalence Principle EP states that inertial mass is equal to gravitational (or heavy) mass.
In Newtonian mechanics EP can be seen as a definition of inertial mass in terms of gravitational mass and a definition of force in terms of gravitational force via Newton’s 2nd Law
- Force = Mass x Acceleration, or F = M x A .
It works this way: Given a gravitational force field, its strength G is defined to be equal to the acceleration A of any body falling freely subject to a gravitational force F = G x M, where M is gravitational mass M in accordance with Newton’s 2nd Law. Inertial mass is then defined to be equal to gravitational mass M and a general force F is defined in terms of inertial mass M and A by F = M x A. Both gravitational and inertial mass are thus defined by weight.
In short, in Newtonian mechanics inertial mass and general force are defined in terms of gravitational mass and gravitational force, through Newton’s 2nd Law, with Newton’s 2nd Law serving as a physical law expressing that all bodies fall equally fast in a gravitational field and as a definition for general forces and motion. In particular, EP is true by definition.
General relativity is based on EP (combined with general covariance) and EP is similarly true by definition. The physics of general relativity enters through the stress/energy source in Einstein’s equations, and not through EP. Nevertheless, physicists consider EP to express a physical law, which in principle may be true or false, but a physical law which in general relativity is true by definition.
EP thus is viewed as both a physical law and as a definition. The same ambiguity is inherent in relativity theory which makes it difficult to refute. You cannot refute a definition.
To deny or even question that there are 100 centimeters on a meter, is both silly and impossible.