 The first response says: so much the worse for tropes and immanent universals. This response uses a mereo-locational principle, No Interpenetration, as a premise in an argument against certain metaphysical views, namely those that posit tropes or immanent universals. Is there some reason why mereo-locational principles should not be used in this way? The principles of pure mereology are often so used. For example, Lewis (1999: 108–110) rejects states of affairs and structural universals on the grounds that they would violate the Uniqueness of Composition, the principle that no entities xx have more than one  fusion.[5]  Why not give the same status to certain mereo-locational principles? Why not, for example, say that No Interpenetration is better justified than is the view that universals or tropes are spatiotemporally located? True, it is hard to say much by way of argument for No Interpenetration. But the same can be said of various purely mereological principles, such as the Transitivity of Parthood, and this is often treated as a non-negotiable constraint that any metaphysical position must obey.