 There are arguments on both sides (van Fraassen 1997). Those who argue for the necessity of such a dynamics maintain that we have to assure that the trajectories of actual properties really are, at least for macroscopic objects, like we see them to be, i.e., like the records contained in memories. For example, we should require not only that the book at rest on the desk possesses a definite location, but also that, if undisturbed, its location relative to the desk does not change in time. Accordingly, one cannot get away with simply specifying possible definite properties at each instant of time. We need also to show that this specification is at least compatible with a reasonable dynamics; better still, to specify this dynamics explicitly.