Borders are strange and unique entities, mind-dependent but realised in mind-independent geography. It is this oddness that gives them their allure, drawing us to them. 

Hadrian’s Wall. The Berlin Wall. The Demilitarised Zone (DMZ) between South Korea and North Korea. The ‘Golden Triangle‘ between Thailand, Laos and Myanmar. The ‘Four Corners’ between Arizona, Colorado, New Mexico and Utah. All these borders are tourist attractions, drawing large numbers of visitors every year. Some of them, such as the Berlin Wall, are not even borders today - they mark historical borders. Why do tourists visit them? Why is ‘border tourism’ so popular? Part of the fascination we have for borders lies in what they are: peculiar, mind-dependent things.

Geographical borders exist, but not in an ordinary way. The OED defines such borders as a ‘boundary line which separates one country from another’ . Borders clearly exist: it is a geo-political fact that France borders Germany, that South Korea touches the North. Nations use borders to mark the limits of their sovereignty, to define territories, to monitor or control the flow of people and ideas.

If borders are mind-dependent, we are idealists about them. They are created by human minds, and depend on minds for their existence.

Yet they don’t exist in the way that trees or rocks do. These things exist, and they are accessible to our senses: we can see trees, touch rocks. In contrast, borders need not be accessible to our senses. Anthropologist Tim Ingold makes this point by describing borders as ‘ghostly lines’:

I came across one such line while herding reindeer along the border between Finland and Russia… The border was marked by a clear-cut strip of forest, down the mid-line of which the actual frontier was supposed to run. It was marked in no other way save by occasional posts. Had I attempted to cross it, however, I would have been shot at from one of the many observation towers on the Soviet side.

Some borders trace ordinary things. The Danube river separates Hungary from Slovakia. The Pyrenees mountain range divides France from Spain. But borders are not identical with rivers or mountains: if the Detroit River dried up, the Canada-USA border would remain.

Given this strangeness, what is a border? Ingold’s 2007 Lines: A Brief History describes borders and other ghost lines, such as air-space partitions and time-zones, as ‘imaginary’. When we consider longitudes and latitudes, it is ‘as if’ we stretched a taut string between points. Philosopher Barry Smith describes them as ‘abstract’. "The border of Colorado", he writes, "is an abstract mathematical line… corresponding to no underlying physical reality". Other abstract objects include concepts like ‘justice’ or the number ‘four’ - these are also things we cannot see or touch.

These divides are ‘intriguing’ because they are human constructs, reflecting socio-political realities.

The same basic idea underlies these proposals: borders are mind-dependent. When considering a thing, you can ask whether its existence depends on minds. Realism holds things exist independently of minds. I am a realist about hills, houses, and moons. If conscious beings disappeared from the universe, I believe these things would continue to exist. Idealism, by contrast, holds the existence of things depends on minds. You can be an idealist about all kinds of things. For example, a philosopher of mind might be an idealist about emotion: without minds, there cannot be ‘joy’ or ‘despair’. A philosopher of perception might be an idealist about colour: the world has only light rays and particles, our brain creates reds and blues.