《资治通鉴·汉纪·汉纪九》
有高山为界，行人绝迹，车道不通，这是天地用来限隔中原和边外的自然屏障。
限以高山，人迹绝，车道不通，天地所以隔外内也。
The border is marked by high mountains, where no one goes and no roads pass through. This is a natural barrier that Heaven and Earth have created to separate the Central Plains from the border regions.