 By definition of ‘PP’, all atoms are pairwise disjoint and can only overlap things of which they are part. Are there any such entities? And, if there are, is everything entirely made up of atoms? Is everything comprised of at least some atoms? Or is everything made up of atomless “gunk”—as Lewis (1991: 20) calls it—that divides forever into smaller and smaller parts? These are deep and difficult questions, which have been the focus of philosophical investigation since the early days of philosophy and throughout the medieval and modern debate on anti-divisibilism, up to Kant's antinomies in the Critique of Pure Reason (see the entries on  ancient atomism  and  atomism from the 17th to the 20th century).  Along with nuclear physics, they made their way into contemporary mereology mainly through Nicod's (1924) “geometry of the sensible world”, Tarski's (1929) “geometry of solids”, and Whitehead's (1929) theory of “extensive connection” mentioned in Section 3.1, and are now center stage in many mereological disputes at the intersection between metaphysics and the philosophy of space and time (see, for example, Sider 1993, Forrest 1996a, Zimmerman 1996, Markosian 1998a, Schaffer 2003, McDaniel 2006, Hudson 2007a, Arntzenius 2008, and J. Russell 2008, and the papers collected in Hudson 2004; see also Sobociński 1971 and Eberle 1967 for some early treatments of these questions in the spirit of Leśniewski's Mereology and of Leonard and Goodman's Calculus of Individuals, respectively). Here we shall confine ourselves to a brief examination.