  A second sort of puzzle relates to the fact that Aristotle's mereological definition, and the common-sense intuition that it captures, only seem to apply to the realm of continuous entities. Modulo the above-mentioned difficulty, the thought that Maryland and Pennsylvania are bounded by the Mason-Dixon line is fair enough. But ordinary material objects — it might be observed — are not truly continuous (or dense) and speaking of an object's boundary is like speaking of the “flat top” of a fakir's bed of nails (Simons 1991: 91). On closer inspection, the spatial boundaries of physical objects are imaginary entities surrounding swarms of subatomic particles, and their exact shape and location involve the same degree of idealization of a drawing obtained by “connecting the dots”, the same degree of arbitrariness as any mathematical graph smoothed out of scattered and inexact data, the same degree of abstraction as the figures' contours in a Seurat painting. Similarly, on closer inspection a body's being in motion amounts to the fact that the vector sum of the motions of zillions of restless particles, averaged over time, is non-zero, hence it makes no sense to speak of the instant at which a body stops moving (Galton 1994: 4). All this may be seen as good news vis-à-vis the puzzles of Section 1.1, which would not even get off the ground (at least in the form given above; see S. R. Smith 2007 and Wilson 2008 for qualifications). But then the question arises: Is our boundary talk a mere façon de parler?