"Distance" is a basic concept in classical mathematics. Distance is a description of the proximity and positional relationship between two points or two points and a set or two sets. However, classical mathematics stipulates that the distance between each point in the point set and the point set is 0, which describes "the same within the class"; There is no difference between any two point sets whose intersection is not empty, and the distance between them is 0. In fact, the intersection of two different sets is different, so the distance between them should be different, but the concept of point set distance in classical mathematics does not describe this difference.