“The fact that the Roman numerals were so deeply rooted in the customs and affections of the people at first made it exceedingly difficult for the new Indian numerals, the ‘figure numbers,’ to replace the old familiar Roman numerals. . . . [I]n northern Europe the Indian numerals first began to be used by ordinary people about 1500. This date, the change from the fifteenth to the sixteenth century, is the great intellectual watershed of modern history, the time when all the new movements generally came to the fore.”

—Karl Menninger, Number Words and Number Symbols; A Cultural History of Numbers, 1969.