   The complexities are further amplified if one, like Russell as for Frege, defines even basic objects like natural numbers as classes of classes. For instance the number 2 is defined as the class of all classes of individuals having exactly two elements. We again obtain natural numbers of different orders in the ramified hierarchy. Besides Russell himself, and despite all these complications, Chwistek tried to develop arithmetic in a ramified way, and the interest of such an analysis was stressed by Skolem. See Burgess and Hazen 1998 for a recent development.