_Rule_ XLI. Though in the question about the obligation of an oath that is taken ignorantly, or by deceit, there be great difficulties, yet this much seemeth clear: 1. That he that is culpably ignorant is more obliged by his vow or contract while he useth all the outward form, than he that is inculpably ignorant. 2. That though the deceit (as the force) of him that I swear to, do forfeit his right to what I promise him, yet my oath or vow obligeth me to do or to give the thing, having interested God himself in the cause. 3. That all such errors of the essentials of an oath or vow as nullify it, (of which I spake before,) or make the matter sinful, do infer a nullity in the obligation (or that it must not be kept). But no smaller error (though caused by deceit) doth disoblige.
The commonest doubt is, Whether an error about the very person that I swear to, and this caused by his own deceit, do disoblige me? All grant that I am obliged notwithstanding any circumstantial error (as if I think a woman rich whom I marry, and she prove poor; or wise and godly, and she prove foolish or ungodly: yea, if the error be about any integral part; as if I think she had two eyes or legs, and she have but one): and all grant that an error about an essential part, that is, which is essential to the relation or thing vowed, (if inculpable at least,) disobligeth: as if I took a man in marriage thinking he had been a woman; or if I took a person for a pastor, a physician, a counsellor, a pilot, that hath no tolerable ability or skill in the essentials of any of those professions. But whether I am bound if I swear to Thomas thinking it was John, or if I marry Leah thinking she is Rachel, is the great doubt. And most casuists say I am not: and therefore I dare not be bold to contradict them.[85] But I much suspect that they fetched their decision from the lawyers; who truly say, that _in foro civili_ it inferreth no obligation: but whether it do not oblige me ethically, and _in foro conscientiæ et cœli_, I much doubt,[86] 1. Because it seemeth the very case of Joshua and the Israelites, who by the guile of the Gibeonites were deceived into an _error personarum_, taking them to be other persons than they were: and yet that this oath was obligatory, saith Dr. Sanderson, is apparent, (1.) In the text itself, Josh. ix. 19. (2.) In the miracle wrought for that victory which Joshua obtained in defending the Gibeonites when the sun stood still, Josh. x. 8, 13. (3.) In the severe revenge that was taken on the lives of Saul's posterity for offering to violate it, 2 Sam. xxi. 2. 2. And this seemeth to be the very case of Jacob, who took not himself disobliged from Leah notwithstanding the mistake of the person through deceit. And though the _concubitus_ was added to the contract, that obliged most as it was the perfecting of the contract, which an oath doth as strongly. 3. And the nature of the thing doth confirm my doubt; because when I see the person before me there is the _individuum determinatum_, in the _hæc homo_, and so all that is essential to my vow is included in it: if I mistake the name, or the quality, or birth, or relations of the person, yet my covenant is with this determinate person that is present, though I be induced to it by a false supposition that she is another. But this I leave to the discussion of the judicious.
_Rule_ XLII. The question also is weighty and of frequent use, if a man vow a thing as a duty in obedience to God and conscience, which he would not have done if he had taken it to be no duty, and if he afterwards find that it was no duty, is he obliged to keep this vow? And the true answer is, that the discovery of his error doth only discover the nullity of his obligation to make that vow, and to do the thing antecedently to the vow; but if the thing be lawful, he is bound to it by his vow notwithstanding the mistake which induced him to make it.
_Rule_ XLIII. Vows about trifles (not unlawful) must be kept though they are sinfully made.[87] As if you vow to take up a straw, or to forbear such a bit or sort of meat, or garment, &c. But to make such is a great profanation of God's name, and a taking it in vain as common swearers do.
_Rule_ XLIV. A general oath, though taken upon a particular occasion, must be generally or strictly interpreted (unless there be special reasons for a restraint, from the matter, end, or other evidence). As if you are afraid that your son should marry such a woman, and therefore swear him not to marry without your consent; he is bound thereby neither to marry that woman nor any other. Or if your servant haunt one particular ale-house, and you make him forswear all houses in general, he must avoid all other. So Dr. Sanderson instanceth in the oath of supremacy, p. 195.