  A related problem is the fracture problem, described by Medlin. Imagine fracturing a material body such as a piece of wood, regarded as a plenum (full of matter). What is the state of the two new surfaces after the fracture? Unless matter is to be created or destroyed, we seem to have to say that the break is half-open, with one new matter-surface being topologically closed and the other being topologically open. But which surface is which? There seems to be no principle to determine which. In response, it can be asked how seriously we have to take the postulation of a plenum. If for example matter is as Boscovich suggested, punctate and surrounded by fields, then there are no plena, and the problem is no more than hypothetical. Or again, there might be plena but other principles might apply. For example, mass-density functions might drop smoothly to zero at the boundaries between matter and empty space, which would mean that all surfaces were open. On the other hand, it might be instead that as a matter of fact all surfaces are topologically closed. This would need an inconsistent solution (see below, sections 5–7).