In this post, we put forth our understanding of Natural boundaries between countries. By Natural we approximately mean sustainable and fair.
We first define boundaries between countries A and B. A boundary between A and B is a mechanism for restricting people from moving from A to B in the same way they can move within their own country. This restriction is a physical manifestation of an abstraction. When it's an ocean or a river, it's a physical manifestation of Nature. When it's a fence, it's the physical manifestation of the need to not let people from other countries in without due process.
We now define culture. We approximately define culture to be the set of ideas that people living in geographical proximity believe in.
A boundary that is not a physical manifestation of Nature, like an ocean, a mountain, or a river, assumes that there's a sudden difference between the people from A and B at the boundary, to warrant the existence of the boundary.
This is opposite to how cultures work. There is never a jolting difference between the cultures of two countries, A and B, at any boundary that is non-natural, at least when the boundary is created. We believe that this is the case because human beings have been transferring culture based on geographical proximity, or natural boundaries.