. The converse domain of a relation is the domain of its converse: thus the class of wives is the converse domain of the relation of husband to wife. We may now state our definition of similarity as follows:—
One class is said to be "similar" to another when there is a one-one relation of which the one class is the domain, while the other is the converse domain.
It is easy to prove (1) that every class is similar to itself, (2) that if a class
is similar to a class
, then
is similar to