 Central to any theory of sets is a statement of the conditions under which sets are formed. In addition to simply listing the members of a set, it was initially assumed that any well-defined condition (or precisely specified property) could be used to determine a set. For example, if \(T\) is the property of being a teacup, then the set, \(S\), of all teacups might be defined as \(S = \{x: T(x)\}\), the set of all individuals, \(x\), such that \(x\) has the property of being \(T\). Even a contradictory property might be used to determine a set. For example, the property of being both \(T\) and not\(-T\) would determine the empty set, the set having no members. 