Gunk (mereology)

In mereology, an area of metaphysics, the term gunk applies to any whole whose parts all have further proper parts. That is, a gunky object is not made of indivisible atoms or simples. Because parthood is transitive, any part of gunk is itself gunk. The term was first used by David Lewis in his work Parts of Classes (1991),^[1] in which he conceived of the possibility of "atomless gunk",^[2] which was shortened to "gunk" by later writers. Dean W. Zimmerman defends the possibility of atomless gunk.^[3]

If point-sized objects are always simple, then a gunky object does not have any point-sized parts, and may be best described by an approach such as Whitehead's point-free geometry. By usual accounts of gunk, such as Alfred Tarski's in 1929,^[4] three-dimensional gunky objects also do not have other degenerate parts shaped like one-dimensional curves or two-dimensional surfaces.

Gunk is an important test case for accounts of the composition of material objects: for instance, Ted Sider has challenged Peter van Inwagen's account of composition because it is inconsistent with the possibility of gunk. Sider's argument also applies to a simpler view than van Inwagen's: mereological nihilism, the view that only material simples exist. If nihilism is necessarily true, then gunk is impossible. But, as Sider argues, because gunk is both conceivable and possible, nihilism is false, or at best a contingent truth.^[5]

Gunk has also played an important role in the history of topology^[6] in recent debates concerning change, contact, and the structure of physical space. The composition of space and the composition of material objects are related by receptacles—regions of space that could harbour a material object. (The term receptacles was coined by Richard Cartwright.)^[7] It seems reasonable to assume that if space is gunky, a receptacle is gunky and then a material object is possibly gunky.