Cicero, Pro Quinctui Roscio 16
“Still,” he said, “Cluvius told Lucius and Manilius he was not on sworn oath.” If he told them while sworn in, would you believe? What is the difference between a perjurer and a liar? A man who is accustomed to lying, can get used to committing perjury.
I can easily get a man to perjure himself once I am able to persuade him to lie. For once someone has departed from the truth, he is not in the habit of being constrained by greater belief from perjury than from lying. For what man who is not moved by the force of his own conscience is moved by invocation of the gods?
The reason for this is that the gods dispense the same penalty for the perjurer and the liar. The gods become enraged and punish a man not for the institution which frames the swearing of the words but because of the evil and the malice that these traps are set for another person.”
XVI. “Dicit enim,” inquit, “iniuratus Luscio et Manilio.” Si diceret iuratus, crederes? At quid interest inter periurum et mendacem? Qui mentiri solet, peierare consuevit. Quem ego, ut mentiatur, inducere possum, ut peieret, exorare facile potero. Nam qui semel a veritate deflexit, hic non maiore religione ad periurium quam ad mendacium perduci consuevit. Quis enim deprecatione deorum, non conscientiae fide commovetur? Propterea, quae poena ab dis immortalibus periuro, haec eadem mendaci constituta est; non enim ex pactione verborum, quibus ius iurandum comprehenditur, sed ex perfidia et malitia, per quam insidiae tenduntur alicui, di immortales hominibus irasci et suscensere consuerunt.