. This relation, like the relation
, is by no means capable of being identified with the inductive cardinal number
, because a relation and a class of classes are objects of utterly different kinds.[15] It will be seen that
is always the same relation, whatever inductive number
may be; it is, in short, the relation of 0 to any other inductive cardinal. We may call this the zero of rational numbers; it is not, of course, identical with the cardinal number 0. Conversely, the relation
