 An attractive feature of this approach is that it provides an answer to Hempel’s dilemma and similar problems. Physics may indeed change over time, but according to the structuralist, any physical theory must be restricted to this sort of vocabulary. Hence we can appeal to the notion of a physical theory to formulate a version of physicalism on which everything supervenes on or is realised by or is grounded in physical properties that can be expressed in that limited vocabulary. (Here structuralism about the physical draws support from structural realism in philosophy of science, a position to which it is in some ways quite similar; see, e.g.  Structural Realism)