is spouse of
. Suppose now we are given the relation spouse, and we wish to derive the relation husband. Husband is the same as male spouse or spouse of a female; thus the relation husband can be derived from spouse either by limiting the domain to males or by limiting the converse to females. We see from this instance that, when a symmetrical relation is given, it is sometimes possible, without the help of any further relation, to separate it into two asymmetrical relations. But the cases where this is possible are rare and exceptional: they are cases where there are two mutually exclusive classes, say
and
, such that whenever the relation holds between two terms, one of the terms is a member of
and the other is a member of