 The most general comprehension axiom of Z is the axiom of Separation. If we try to replicate the Russell paradox by constructing the set \(R' = \{x \in A \mid x \not\in x\}\), we discover that \(R' \in R' \leftrightarrow R' \in A \amp R' \not\in R'\), from which we deduce \(R' \not\in A\). For any set \(A\), we can construct a set which does not belong to it. Another way to put this is that Z proves that there is no universal set: if we had the universal set \(V\), we would have naive comprehension, because we could define \(\{x \mid P(x)\}\) as \(\{x \in V \mid P(x)\}\) for any property \(P(x)\), including the fatal \(x \not\in x\).