“Everyone can see . . . sets of one, of two, and of three objects . . . and most people can see . . . set[s] of four. . . . Beyond four, quantities are vague, and our eyes alone cannot tell us how many things there are. . . .

The eye is simply not a sufficiently precise measuring tool: its natural number-ability virtually never exceed four. . . .

Perhaps the most obvious confirmation of the basic psychological rule of the ‘limit of four’ can be found in the almost universal counting-device called (in England) the ‘five-barred gate’. It is used by innkeepers keeping a tally or ‘slate’ of drinks ordered, by card-players totting up scores, by prisoners keeping count of their days in jail. . . .”

—Georges Ifrah, The Universal History of Numbers, 2000.