These questions were posed by professors Roberto Casati and Achille C. Varzi in their 263 page book on the subject of ‘Holes and Other Superficialities’ (The MIT Press, 1994). And considerable steps were made towards answers.
The duo took a comprehensive look into the myriad of real-world hole-like manifestations and distilled them down into just three basic types :
• Blind Hollows,
• Perforating Tunnels, and
• Internal Cavities.
“The relevant notion operating here is that of filling: objects with holes – or, better, holes in objects – constitute the morphological manifold of fillable entities.” explain the authors.
More detailed information on the ontological, mereological, topological, and morphological aspects of holes is available here:
BONUS: In the interests of furthering hole research, Improbable invites readers to submit examples of holes which appear not to fit into the above categories of blind hollows, perforating tunnels, or internal cavities.
Also see : Almost Nothings (Improbable Research, May 26th, 2011)