 After all, what is it that keeps material objects from interpenetrating in the actual world? Repulsive forces, presumably. But a standard view is that the laws governing such forces are not metaphysically  necessary.[6]  And on that assumption it is natural to conclude that there are metaphysically possible worlds in which any repulsive forces that exist can be overridden in such a way as to allow material objects to interpenetrate. (For more on this, see Zimmerman 1996a and Sider 2000.)