Editing Naive set theory (section)

=== Membership ===
If ''x'' is a member of a set ''A'', then it is also said that ''x'' '''belongs to''' ''A'', or that ''x'' is in ''A''. This is denoted by ''x''&nbsp;∈&nbsp;''A''. The symbol ∈ is a derivation from the lowercase Greek letter [[epsilon]], "ε", introduced by [[Giuseppe Peano]] in 1889 and is the first letter of the word [[:wikt:ἐστί|ἐστί]] (means "is"). The symbol ∉ is often used to write ''x''&nbsp;∉&nbsp;''A'', meaning "x is not in A".