Full Idea
In the iterative conception of sets, they form a natural hierarchy.
Clarification
In 'iteration', each one generates the next one
Gist of Idea
In the iterative conception of sets, they form a natural hierarchy
Source
Chris Swoyer (Properties [2000], 4.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.16