 In more formal terms, the problem arises because we introduced force \(\bF\) and mass \(m\) as \(T\)-theoretical terms that are not given by other theories. That fact also supplies an escape from the problem. We can add extra laws to the simple dynamics. For example, we might require that all forces are gravitational and that the net force on the mass \(m\) be given by the sum \(\bF=\Sigma_i \bF_i\) of all gravitational forces \(\bF_i\) acting on the mass due to the other masses of the universe, in accord with Newton’s inverse square law of gravity. (The law asserts that the force \(\bF_i\) due to attracting mass \(i\) with gravitational mass \(m_{gi}\) is \(Gm_g m_{gi} \boldsymbol{r}_i / r_{i}^3\), where \(m_g\) is the gravitational mass of the original body, \(\boldsymbol{r}_i\) the position vector of mass \(i\) originating from the original body, and \(G\) the universal constant of gravitation.) That gives us an independent definition for \(\bF\). Similarly we can require that the inertial mass \(m\) be equal to the gravitational mass \(m_g\). Since we now have independent access to each of the terms \(\bF\), \(m\) and \(\ba\) appearing in \(\bF=m\ba\), whether the law obtains is contingent and no longer a matter of definition.