Fiat and Bona Fide Boundaries

There is a basic distinction, in the realm of spatial boundaries, between bona fide boundaries on the one hand, and fiat boundaries on the other. The former are just the physical boundaries of old. The latter are exemplified especially by boundaries induced through human demarcation, for example in the geographic domain. The classical problems connected with the notions of adjacency, contact, separation and division can be resolved in an intuitive way by recognizing this two-sorted ontology of boundaries. Bona fide boundaries yield a notion of contact that is effectively modeled by classical topology; the analogue of contact involving fiat boundaries calls, however, for a different account, based on the intuition that fiat boundaries do not support the open/closed distinction on which classical topology is based. In the presence of this two-sorted ontology it then transpires that mereotopology—topology erected on a mereological basis—is more than a trivial formal variant of classical point-set topology. 1. Parts and Boundaries Consider John, the moon, a lump of cheese. These are objects possessed of divisible bulk. They can be divided, in reality or in thought, into spatially extended parts. They have interiors. They also have boundaries, which we can think of (roughly) as infinitely thin extremal slices. The boundary of the moon is the lunar surface. The boundary of John is the surface of his skin. But what of inner boundaries, the boundaries of the interior parts of things? There are many genuine two-dimensional (sphereand torus-like) boundaries within the interior of John’s body in virtue of the differentiation of this body into organs, cells, and so on. Imagine, however, a spherical ball made of some perfectly homogeneous prime matter. If the possession