Look at human knowledge as a landscape.  The river basins are the big areas of thought and creativity: medicine, history, art, music.  The knowledge landscape, initially flat and featureless, acquired streams over the course of time; these grew larger, and gained new tributaries as they eroded back into the underlying rocks.  The course of the rivers was partly determined by the geology, but partly also by luck, as bigger rivers began to capture the headwaters of other waterways and to dominate them (science has been very activeof late; theology is fast becoming a desert).
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Once the valleys were formed, they became difficult to alter; all streams eventually turn into tributaries of one or other major river system.  No stream can cross a watershed.  Only the construction of dams and canals can change the hydrology.
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The existence of these knowledge-based boundaries has implications that mirror the implications of physical boundaries in wider politics.  Departmental fiefdoms in universities have analogies with feudal ones.  But the history of political boundaries, and the development of the nation state, shows that the modern concept of borders is not inevitable or natural in early stages of development.  Early empires had spheres of influence.  They demanded tribute or allegiance of peoples within those areas.  But boundaries as such were not important: even to acknowledge their existence, would have been to admit that the imperial power no longer held sway beyond them. 
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The Greeks thought of themselves as a people speaking the same language, united by a shared culture; the boundaries between themselves and the barbarians were mental rather than physical.  The Romans only started building boundary walls when the empire was under threat, or in retreat: the Limes, Hadrian’s wall.  Jupiter Terminus was god of the boundary stones of fields, a type of demarcation that anticipated and perhaps formed the basis of the later notion of national boundaries.  Field boundaries were important in all agricultural societies where private onwnership exists; biblical penalties for moving them were severe.  But nations defended their territory by power, not fences.
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The paradigm boundary is the Great Wall.  This was in fact never fully completed as a continuous national boundary in the modern sense.  It could be seen more as a chain of fortified posts, capable of mutual reinforcement.  Its meandering course along the mountainous parts of its route appears to have been designed so that no part of the wall can be overlooked from either side: so that even if outflanked, it still could be defended.  Its route always runs up and down the line of steepest slope, which means that it runs at right angles to the contour lines at every point.  Therefore it connects the highest points along its route, ensuring that it dominates the landscape in both directions.  The Art of War emphasises the importance of controlling the high ground: the French failure to follow this advice at Dien Bien Phu gave General Giap the opportunity to gain the victory that ended in colonial defeat in Vietnam.
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During the anti-Japanese war in China, whenever communist guerillas entered a village they tore down the mud walls traditionally used for defence against bandits.  This ensured that the Japanese could never bottle them up inside the village by controlling the gates.  This need to demolish walls became something of an ideological habit: the city walls of Beijing were demolished, in an act of vandalism anticipating the Cultural Revolution. 
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But the Chinese, and Roman walls were isolated examples for their day.  The notion of power was a complex of personal rights such as ownership of property (in which fiefdoms were often acquired by marriage, and seen as personal wealth), valour in war and strength of personality.  A king could control the areas he could control; these comprised his country.  Arguably, boundaries could only be clearly defined once technology had advanced sufficiently to allow maps to be drawn up on agreed principles.  Boundaries then became the means by which power could be demonstrated or challenged, and of importance in national defence.
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In academia, boundaries are seldom crossed; yet the most original work is often done by boundary-crossers.  This mutual isolation is reinforced by the equivalent of distinct national languages.  The sciences require knowledge of mathematics: the Sanscrit of knowledge, which has prestige, an ancient lineage and a holy aura of dread for the uninitiated.  Sociology also has its own language, and is even more akin to the mediaeval church than the sciences.  The sacred texts (Marx, Althusser) are carefully analysed and their implications are teased out, but the uninitiated sometimes find it hard to understand these discussions.
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In order to survey the knowledge landscape and find new connections, there is a need to go to the high ground, not stay in the valleys; the Art of War has something to say about the arts of peace.  Species which go to far down the path of overelaboration tend to become extinct; evolution favours the generalist, in the long term.  We need boundaries to make sense of the world, just as we need words to have stable meanings if conversation is to be possible; but we also need to allow the meanings of words to change if necessary over time, and boundaries need to be flexible as well. 
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Wall, or watershed: let us look at boundaries as a challenge.  Anyone for a bit of canal-digging?