element

The objects are called elements or members of the set.

… * b):

…the class x has no members, and x ≠ Λ means that x has at least one member.

…if they contained the same elements. So in his opinion there was only one empty set (today symbolized by Ø), the set with no members. A second set could be defined as having only one element by letting that element be the empty set itself (symbolized by {Ø}), a set…