When — as Quadrigarius says of the Lucanians, in Gellius (bk. 1, ch. 19) — the Israelites found out [that] “words had been given to them by deceits,” was it necessary for them to stand by the pacts and agreements? There are two extreme opinions, and between them two middle [ones]. Of these, the former [extreme] is [that] it is lawful [fas], with enemies, to swear and to forswear [break oaths], as one pleases. This is handed down [to have been the view] of Gentili’s — I know not whose — Lysander, who, on Plutarch’s testimony, was wont to say: “boys are to be deceived with knucklebones [dice], but men with oaths.” The latter [extreme] is Gratian’s: [that] that unlawful oath is to be kept, which [concerns] a thing not evil in itself, but yet forbidden by some positive precept — which [precept] in general indeed was known to be obeyed, but in particular was not known [to apply] in that matter which was sworn; so that it is not ignorance of law, but of fact (causa 22, q. 4). “But since by all these [arguments] unlawful oaths are proved not to be kept, [the objection] is raised, that Joshua did not destroy the Gabaonites, against the Lord’s precept, because the elders of Israel had confirmed peace with them by oath. But it is to be noted, that the elders of Israel, although they knew [that] it was commanded by the Lord that they destroy the nations of the land of promise, yet — deceived by the Gabaonites — did not know [that] these were inhabitants of the land promised to them.” And afterward: “Therefore the unlawful oath which is forbidden to be kept is to be understood [as that] which is known to be unlawful when it is sworn; but if it is not known to be unlawful, ignorance itself excuses — if it be thought to be lawful — [ignorance] not of law, but of fact.” The third opinion, and the former of the middle [two], is Cajetan’s: [that] this oath was lawful, and [that] the Israelites were bound by it, so that they were held to keep [it]: “The chief men,” he says, “who had sworn, judged the oath ought to be kept, out of reverence for the divine name. And deservedly, because, although deceived, they nevertheless had to admit the Gabaonites [as] not hardened in the worship of idolatry, but prepared for the worship of the Most High God, and fearing his judgments.” For [that] the divine decree (Exod. 34 and Num. 33) is to be understood of the obstinate, is made manifest below in ch. 11, where the reason for the disparity between the Gabaonites and the other peoples is rendered — [resting] on the hardening of the others, and the non-hardening of the Gabaonites. And so the oath was not against the decrees of God, and therefore was to be kept — although they erred in the manner of covenanting, because they consulted not the mouth of the Lord. The latter of the middle opinions is [that] the Israelites were not bound by that sacrament [oath], because a fraud was done to them by the Gabaonites. For since these [Gabaonites] had said [that] they dwelt far outside the borders of the land of Canaan, the mind of all who swore was directed toward such [men], and [men] so remote and foreign.
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Since, therefore, the Gabaonites were not such [as they claimed], the Israelites were bound to them in nothing. Their excessive cunning is indeed disapproved — [those] who in words indeed swear one thing, but interpret [it] otherwise in mind, and think themselves to bear a mind unsworn; but these our [Israelites] swear this very [thing] which they understand in mind and decree [to be] true, and nothing else, by word. But the matter is otherwise [than the Gabaonites represented], and therefore they were wholly unbound. “Error in the substance [corpus] vitiates the contract” (Law 19, On the condition of purchase). But in the substance of this contract and covenant there was a great error — and that indeed of those who were covenanting, admitted [introduced] by the fraud of the Gabaonites. The common rule is [that] “a promise does not bind the parties under the good faith of a false assertion.” And “in a promise extorted by guile, and made through error, the oath effects nothing, nor does it supply the defect of consent. An oath which has no consent, has no spirit. Where consent fails, there [fails] also the law of nature.”