  The puzzle can be generalized. Consider the dilemma raised by Leonardo in his Notebooks: What is it that divides the atmosphere from the water? Is it air or is it water? (1938: 75–76). Or consider Suárez's worry in Disputation 40 (Sect. V, §58), repeatedly echoed by Peirce (1892: 546; 1893: 7.127): What color is the line of demarcation between a black spot and its white background? Perhaps figure/ground considerations could be invoked to provide an answer in this second case, based on the principle that the boundary is always owned by the figure — the background is topologically open (Jackendoff 1987, App. B). But what is figure and what is ground when it comes to two adjacent halves of the same black spot? What is figure and what is ground when it comes to Maryland and Pennsylvania? What happens when we dive into the water? Besides, it would be natural to suppose that all entities of the same sort be treated alike, for instance, that all material bodies be construed as figure-like entities, each possessing its own boundary. But then, how could any two of them ever come into contact, short of penetrability? (In this last form, the question is widely discussed in recent literature; see e.g. Kline and Matheson 1987, Hazen 1990, Zimmerman 1996b, Kilborn 2007.)