 The formula \(\exists y(x \in y \amp y \in a)\), where \(a\) is a set, does not mention sethood, has only the set \(a\) as a parameter, and is true only of sets by the Axiom of Elements (any witness \(y\) belongs to the set \(a\), so \(y\) is a set, and \(x\) belongs to the set \(y\), so \(x\) is a set). Thus Union is true for sets.