  Eliminativist theories move from the idea that talk of boundaries involves some sort of abstraction — an idea that can be found already in the medieval and modern debate on anti-indivisibilism (Zimmerman 1996a; Holden 2004) and that has recently become the focus of renewed attention in connection with the hypothesis that the universe might consist of “atomless gunk” (Lewis 1991: 20), i.e., that either space and time or matter, or both, divide forever into smaller and smaller parts (Sider 1993, Zimmerman 1996b; Hawthorne and Weatherson 2004; McDaniel 2006; Hudson 2007; J. T. Russell 2008; Arntzenius 2008). What sort of abstraction would be involved here? And how can one account for our ordinary and mathematical talk about boundaries if these are to be explained away as fictional abstractions? With special reference to the boundaries of spatio-temporal particulars, we may distinguish two main approaches.