Most of this Companion is rightly concerned with the theories and practices of professional mathematicians. But all human beings have ideas about numbers, space, and shape, and ways of putting these ideas to use. It could be said that numeracy is to mathematics what literacy is to literature: everyday, routine application versus expert, elite innovation. But while literacy is now a wildly fashionable subject of academic study, the word “numeracy” is not even recognized by my mass-market word processor. Yet an array of interesting work has been done on nonprofessional mathematical concepts, practices, and attitudes. They range from historical studies and ethnographies to cognitive analyses and developmental ...