The axioms for Zermelo-Fraenkel (ZF) set theory are an appealing but somewhat arbitrary-seeming assortment. A survey of the axioms does not suffice to reveal the source of their attraction. Accordingly, attempts have been made to ground ZF in principles whose appeal can be felt immediately. These attempts can be classified as follows. First, some of them propose to rest the ZF axioms directly on informal doctrine. The others propose to ground the ZF axioms in other formal axioms that can be regarded as more basic. When the latter approach is taken, ZF continues to draw on informal support, but the draft is made at a more basic level.

The same research can be classified in another way, according to the informal diagnosis offered for the paradoxes of set theory. In some cases, the diagnosis is that the paradoxical sets (such as the Russell set) fail to exist only because they would have to be too large; a set that would be sufficiently small must always exist. This is the doctrine of limitation of size. In other cases, the diagnosis is that the paradoxical sets fail to exist only because they would have to lie too high in a certain hierarchy of sets; a set that would lie sufficiently low in the hierarchy must always exist. This is the doctrine of the hierarchy. The present paper will investigate the latter doctrine, with the doctrine of size making a brief appearance at the end.