  Assume that we have two individuals \(a\) and \(b\) such that \(Q(a, b)\) holds. We can define \(P(x)\) to be \(Q(x, a)\). It is then clear that \(P(a)\) holds, since it is \(Q(a, a)\). Hence \(P(b)\) holds as well. We have proved, in an impredicative way, that \(Q(a, b)\) implies \(Q(b, a)\).