The second major difficulty is along the same lines, concerning, not functions, but relations, and thus ordering relations and ordinal numbers. As we have seen (in §2.2.4), Zermelo has the beginnings of an answer to this in his second proof of the WOT, for this uses a theory of subset-orderings to represent the underlying ordering of a set. It turns out that the method given in this particular case suggests the right way to capture the general notion.