Pufendorf: On the Duty of Man and Citizen according to Natural Law

1. An oath is held to lend conspicuous support to our speech and to all acts which involve speech. For it is a religious affirmation, by which we give up our claim to God's mercy or call down divine punishment if we shall not speak truth. An oath raises a presumption of truth by invoking an omniscient and omnipotent witness and avenger, because we find it hard to believe that anyone would be so impious as boldly to call down upon himself God's most heavy wrath. This is the reason why the duty of those who swear is to take the oath with reverence and to observe scrupulously what they have sworn.

2. Now the most important use and purpose of swearing is to bind men more strongly to tell the truth or to keep a promise or agreement, through fear of God who is omniscient and omnipotent. If they knowingly deceive Him, they invoke His vengeance upon themselves by swearing, when otherwise the fear that threatens them from men would not have been effective, since they would expect to be able to disregard or deflect men's devices or elude detection.

3. Since God alone is omniscient and omnipotent, it is absurd to take an oath by any object which is not believed to be divine in the sense of invoking it as witness and avenger of perjury. It often happens, however, that in taking an oath a certain object is named by which one will swear in the sense that if the swearer breaks his oath, God is to take particular vengeance on that object, since it is something very dear and valuable to the swearer.

4. In oaths, the formula by which God is described in the invocation to Him as witness and avenger must be adapted to the conviction or religious belief which the oath-taker has of God. For there is no compulsion in an oath by a god in whom he does not believe and therefore does not fear. But no one believes he is swearing by God except in the formula or under the name contained in the precepts of his own religion, which is in his view the true one.