 However, it may still be insisted that a threshold age does not mark a significant enough difference. A 40-year-old differs greatly from a 4-year-old. Someone who is 18 years and 1 month does not differ greatly from someone who is 17 years and 11 months. It is understandable that the 40-year-old should have rights whereas the 4-year-old should not. But this is not the case for the latter pairing. This is a point about the extent to which real differences between classes are displayed by the members of each class at the edge of these classes. The reply will be that the criticism concedes a difference between being too young to have rights and being old enough to have them. These differences are not arbitrary. Moreover a threshold has to be fixed. The fact that there may not be significant—or significant enough—differences between the members of the two classes being distinguished at the edges of each class is the price one pays for having to operate with a threshold.