 Empty Set: If the empty set were a proper class, then all proper classes would be empty. In particular, the Russell class would be empty. Let \(I\) be an infinite set. \(\{I\}\) would be a set, because it is not empty, and \(\{I,\{I\}\}\) would be a set (again because it is not empty). But \(\{I,\{I\}\}\) belongs to the Russell class (as a set with two elements, it cannot be either the Dedekind infinite \(I\) or the singleton \(\{I\}\). So \(\varnothing\) is a set.