 On the second of these two points: the idea – what Butterfield (2007) calls ‘Brown’s moral’ – is that one cannot simply posit a piece of geometrical structure in one’s theory, e.g. a Minkowski metric field in STR, and know ab initio that material bodies (in particular rods and clocks) will read off intervals of that structure; rather, whether this is the case or not will depend upon the constitution of, and dynamics governing, those material bodies. We will see below specific theories in which any such assumption seems to fail. Note that this second point is again consistent with the explanatory contention taken above to be characteristic of the dynamical approach: a given piece of structure inherits its operational significance as spacetime by dint of the behaviour of material bodies.