 When Whitehead and Russell logicized the concept of number, their starting point was our intuition of equinumerous classes of individuals—for example, our recognition that the class of dwarfs in the fairy tale of Snow White (Doc, Grumpy, Happy, Sleepy, Bashful, Sneezy, Dopey) and the class of days in a week (from Monday to Sunday) have ‘something’ in common, namely, the something we call ‘seven.’ Then they logically defined (i) classes C and C′ to be equinumerous when there is a one-to-one relation that correlates each of the members of C with one member of C′, and (ii) the number of a class C as the class of all the classes that are equinumerous with C.