Once adjacency has been stipulated, a distance function can be easily derived: the distance between \(p\) and \(q\), \(d(p,q)\), is the smallest number of “links” in a chain of points connecting \(p\) and \(q\) such that each one is adjacent to the previous one. Next there is no problem to show that a straight line passing through two points is that chain of points that has the shortest distance.