 The core idea is that it is possible that a duplicate of our cosmos Cos could be embedded in a still larger world \(w^+\). If priority monism is necessary then it holds at \(w^+\) so Cos (being a mere proper part of \(w^+\) is not fundamental at \(w^+\). But if Cos is not fundamental at \(w^+\), and fundamentality never differs between duplicates then the cosmos is not actually fundamental at our world, since it has a non-fundamental duplicate at \(w^+\).