
Boundary (SEP)
We think of a boundary whenever we think of an entity demarcated from its surroundings. There is a boundary (a line) separating Maryland and Pennsylvania. There is a boundary (a circle) isolating the interior of a disc from its exterior. There is a boundary (a surface) enclosing the bulk of this apple. Sometimes the exact location of a boundary is unclear or otherwise controversial (as when you try to trace out the borders of a desert, the edges of a mountain, or even the boundary of your own body). Sometimes the boundary lies skew to any physical discontinuity or qualitative differentiation (as with the border of Wyoming, or the boundary between the upper and the lower halves of a homogeneous sphere). But whether sharp or blurry, natural or artificial, for every object there appears to be a boundary that marks it off from the rest of the world. Events, too, have boundaries — at least temporal boundaries. Our lives are bounded by our births and by our deaths; the soccer game began at 3pm sharp and ended with the referee’s final whistle at 4:45pm. It is sometimes suggested that even abstract entities, such as concepts or sets, have boundaries of their own (witness the popular method for representing the latter by means of simple closed curves encompassing their contents, as in Euler circles and Venn diagrams), and Wittgenstein could emphatically proclaim that the boundaries of our language are the boundaries of our world (1921: prop. 5.6). Whether all this boundary talk is coherent, however, and whether it reflects the structure of the world or simply the organizing activity of our mind, or of our collective practices and conventions, are matters of deep philosophical controversy.
1. Issues
Euclid defined a boundary as “that which is an extremity of anything” (Elements, I, def. 13). Aristotle defined the extremity of a thing as “the first thing beyond which it is not possible to find any part [of the given thing], and the first within which every part is” (Metaphysics, V, 1022a4–5). Together, these two definitions deliver the classic account of boundaries, an account that is both intuitive and comprehensive and offers the natural starting point for any further investigation into the boundary concept. Indeed, although Aristotle’s definition concerned primarily the extremities of spatial entities, it applies equally well in the temporal domain. Just as the Mason-Dixon line marks the boundary between Maryland and Pennsylvania insofar as no part of Maryland can be found on the northern side of the line, and no part of Pennsylvania on its southern side, so “the now is an extremity of the past (no part of the future being on this side of it), and again of the future (no part of the past being on that side of it)” (Physics, VI, 233b35–234a2). Similarly for concrete objects and events: just as the surface of an apple marks its spatial boundary insofar as the apple extends only up to it, so the referee’s whistle marks the temporal boundary of the game insofar as the game protracts only up to it. In the case of abstract entities, such as concepts and sets, the account is perhaps adequate only figuratively. Still, it is telling that one of the Greek words for ‘boundary’, ὅρος, is also a word for ‘definition’: as John of Damascus nicely put it, “definition is the term for the setting of land boundaries taken in a metaphorical sense” (The Fount of Knowledge, I, 8). Likewise, it is telling that in point-set topology the standard definition of a set’s boundary (from Hausdorff 1914, §7.2) reflects essentially the same intuition: the boundary, or frontier, of a set A is the set of those points all of whose neighborhoods intersect both A and the complement of A (where a neighborhood of a point p is, intuitively, a set of points that entirely “surround” p). It is not an exaggeration, therefore, to say that the Euclidean-Aristotelian characterization captures a general intuition about boundaries that applies across the board. Nonetheless, precisely this intuitive characterization gives rise to several puzzles that justify philosophical concern.
Read the rest here: https://plato.stanford.edu/entries/boundary/






















