 The idea is that the order of the relation \(\lambda x \lambda y (x \in \{x\} \amp y \in \{y\})\) determines the first and second element of the ordered pair. It is a relation in extension, which is the analogue of a property in extension or class. A relation in extension has a distinction between the first and second elements due to the order of the defining relation. The closest in contemporary language would be: