 Singleton: If any singleton \(\{x\}\) is a proper class, then all singletons are proper classes, and the Russell class is a singleton. \(\{I, \varnothing \}\) is a set (both elements are sets, and the class is not a singleton) which cannot be a member of itself, and so is in the Russell class. But so is \(\varnothing\) in the Russell class; so the Russell class is not a singleton, and all singletons are sets.