Fiat and bona fide boundaries

Abstract

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) asinfinitely thin extremal slices. The boundary of the moon is its 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 (sphere- and 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 by an object of genuine inner boundaries presupposes either some interior spatial discontinuity or qualitative heterogeneity, then there is a sense in which there are no boundaries to be acknowledged within the interior of such an object at all.