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...All therefore that can be done between two men not subject to civil power is to put one another to swear by the God he feareth: which swearing, or oath, is a form of speech, added to a promise, by which he that promiseth signifieth that unless he perform he renounceth the mercy of his God, or calleth to him for vengeance on himself. Such was the heathen form, Let Jupiter kill me else, as I kill this beast. So is our form, I shall do thus, and thus, so help me God. And this, with the rites and ceremonies which every one useth in his own religion, that the fear of breaking faith might be the greater.
Hobbes, Lev XIV 31
An oath does not make a contract any more obligatory than no oath.
It appears also that the oath adds nothing to the obligation. For a covenant, if lawful, binds in the sight of God, without the oath, as much as with it; if unlawful, bindeth not at all, though it be confirmed with an oath.
Hobbes, Lev XIV 33