" Think of a dentist who has a list of
patients, and for each patient, a list of teeth. It makes perfect sense (and
plenty of money) to hold the patient fixed and vary his teeth-but it makes
no sense at all to hold one tooth fixed and vary the patient. (Although
sometimes it makes good sense to vary the dentist ... ) "

― Douglas R. Hofstadter , Gödel, Escher, Bach: An Eternal Golden Braid