Newton vs. Einstein

I have stated that Newton's mechanics and his theory of gravitation are but approximations to reality and whose limitations are now known . So it might be questionable to use F = ma and F_grav = m M G/r² as basis to any argument as was done above. Einstein was careful to use these expressions only in situations where they are extremely accurate (small speeds compared to c and small gravitational forces). In these cases the inertial and gravitational masses are identical, as shown by experiment. Then he postulated that the same would be true under all circumstances. This statement, while consistent with Newton's equations, cannot, in a strict logical sense, be derived from them.