Where the parties are unequal, friendship is preserved by establishing a proportion that restores equality. In friendships between fellow-citizens (e.g., commercial relations), money is used as a common standard for determining the value of different kinds of products and services so that a balance can be achieved. It is impossible to apply this kind of common standard to some other kinds of friendly relations (e.g., between lovers). Since people differ, the qualifications and contributions of each party tend to differ in most friendships. There is only one fair way to determine the amount and kind of service one friend owes another in payment for some thing or service received — that is, to use the value of the service or thing in the eyes of the recipient as a standard. This is because donors tend to overvalue that which they have done or given, but by the same token, the standard should be the value set by the recipient before he got the thing or service, and not afterwards, when his attitude about it may change.