  While force, however, thus provides the necessary metaphysical  grounds for the existence of genuine motions, it does not, at least  as far as the physicist is concerned, solve the empirical issue of  which bodies can be ascribed genuine motion (Garber 1995, 307; but  see also Lodge 2003). For, as we have seen, the ascription of force  – of vis viva or mv2  – is itself empirically relative to a frame of reference. The  postulation of force makes genuine motion possible; it does not tell  us which bodies are genuinely in motion and which are moved merely  relatively. In practice Leibniz thus counsels that – as in  astronomy – “one can hold the simplest hypothesis  (everything considered) as the true one” (GM II 184/AG 308).  Although the metaphysician can rest assured that true motion must be  absolute, the physicist must therefore be content to work with  relative motions and simplifying assumptions. 