| Summary: | According to common-sense beliefs, there is a mind-independent world consisting of many midsized physical objects which can survive qualitative changes and have all their parts at every time they exist. This thesis is a metaphysical study of the boundaries of such things. The metaphysical feature of boundaries is that they are ontologically dependent entities. Following the work carried out by Franz Brentano and Roderick Chisholm, the existence of a boundary depends upon the existence of something else. However, regarding boundaries of physical objects (surfaces), they hold a mutual ontological dependence but asymmetrical: surfaces rigidly depend upon physical objects, whereas physical objects generically depend upon surfaces. Although a surface is a two-dimensional boundary (without thickness) which cannot take up physical space, it can be found in space where the physical object it belongs to is found; physical objects cannot take physical space unless they have a surface at every time they exist. Once the ontological nature of boundaries is explained, the thesis discusses some boundary-puzzles such as the belongingness of a boundary, vagueness/sharpness, and the bona fide/fiat distinction. It is thus discussed the problem of contact between physical objects, material co-location of objects and their boundaries, temporal persistence, and composition. The last two chapters focus on composition and vagueness. Universalism and nihilism are explained in terms of the boundary they set to determine when composition occurs and when it does not. According to nihilism, there are neither ordinary physical objects nor their surfaces, but only simples. I suggest that if there is any boundary in a nihilist ontology, it must be a sharp one. However, I defend the idea that if we carve in the world enough, we will find some vagueness at its boundaries.
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