1. In case the thing vowed (all things considered) be a thing which God hath forbidden to be done; that is, in case it be a thing in itself evil; but if the thing in itself be a duty, though there be some inseparable sins which we shall be guilty of in the performance, we must not therefore leave the duty itself undone which we have vowed: as if I vow to praise God, and yet am sure that I cannot praise him without a sinful defect of that love and delight in him which is due, I must not therefore forbear to praise him; else we must cast off all other duty, because we cannot do it without some sin. But yet, though in case of unwilling infirmity, we must thus do the duty though we are sure to sin in it, yet in case of any chosen, voluntary sin, which we have an immediate power to avoid, we must rather forbear the duty itself (vowed or not vowed) than commit such a sin; as if I vow to preach the gospel, and am forcibly hindered unless I would voluntarily tell one lie, or commit one sin wilfully for this liberty; I ought rather never to preach the gospel; nor is it then a duty, but become morally impossible to me; as if in France or Spain I may not preach unless I would take Pope Pius's Trent confession or oath. Nay, if those very defects of love, and wandering thoughts, which now inseparably cleave to my best performances, were morally and immediately in my power, and I could avoid them, I ought not electively and by consent to commit them, for any liberty of duty, but rather to forbear the duty itself as no duty to me when it cometh upon such conditions; for then it is supposed that I could serve God better without that duty, because I could love him more, &c.
Yet here is observable a great deal of difference between omissions and commissions. A man may never commit a sin that good may come by it, though he vowed the good; but a man may ofttimes omit that which else would have been his duty, to do some good which he hath vowed; for negative commands bind _semper et ad semper_; but the affirmative do not (at least as to outward duty); therefore in case of necessity a man may himself consent to the present omission of some good, for the escaping of greater, unavoidable omissions another time, or for the performing of a vow or greater duty which is to be preferred.
2. A vow is not to be kept, when the matter of it is unjust and injurious to another (unless you have his consent): as if you vow to give away another man's lands or goods, or to do him wrong by word or deed; or if you vow to forbear to pay him his due, or to do that which you owe him: as if a servant vow to forbear his master's work (unless it be so small an injury as he can otherwise repair); or a husband, or wife, or parents, or children, or prince, or subjects should vow to deny their necessary duties to each other. Here man's right together with God's law doth make it unjust to perform such vows.
3. A vow is as null or not to be kept, when the matter is something that is morally or civilly out of our power to do: as if a servant, or a child, or subject vow to do a thing, which he cannot do lawfully without the consent of his superior: this vow is not simply null, for it is a sinful vow (unless it was conditional). Every rational creature is so far _sui juris_, as that his soul being immediately subject to God, he is capable of obliging himself to God; and so his vow is a real sinful vow, when he is not so far _sui juris_ as to be capable of a lawful vowing, or doing the thing which he voweth. Such a one is bound to endeavour to get his superior's consent, but not without it to perform his vow; no, though the thing in itself be lawful. For God having antecedently bound me to obey my superiors in all lawful things, I cannot disoblige myself by my own vows.