in various ways, of which the following is perhaps the simplest: Form first all the ordered couples whose first term is a class consisting of a single member of
, and whose second term is the null-class; then, secondly, form all the ordered couples whose first term is the null-class and whose second term is a class consisting of a single member of
. These two classes of couples have no member in common, and the logical sum of the two classes will have
terms. Exactly analogously we can define
, given that