Before taking their seats in Parliament, all MPs must swear an oath of allegiance to the Queen. Mark Durkan, MP for Foyle in Northern Ireland, recently suggested that they should also swear an oath to do no wrong. In this election year, that could set a useful precedent.

Political orators in the Greek world talked of the good citizen as one who cared for the like-mindedness of all citizens and for his city’s interests — defending the fatherland, obeying the laws and authorities, and honouring the state’s cults — and young Athenian males swore to this effect when they reached the...