covenants, or oaths may _quoad effectum_ be reputatively null; and he that by putting his sword to another man's breast doth compel him to swear or subscribe and seal a deed of gift, may be judged to have no right to it, but to be punishable for the force; but though this covenant or promise be null _in foro humano_, because the person cannot acquire a right by violence, yet the oath is not a nullity before God; for when God is made a party, he hath a right which is inviolable; and when he is appealed to or made a witness, his name must not be taken in vain. 3. It is a nullity reputatively when the person is naturally incapable of self-obligation, as in infancy, when reason is not come to so much maturity as to be naturally capable of such a work; I say naturally incapable, for the reasons following. _Rule_ XXVI. We must distinguish between a natural incapacity of vowing or swearing at all, and an incapacity of doing it lawfully; and between a true nullity, and when the oath is only _quasi nullum_, or as null _quoad effectum_, or such as I must not keep. There are many real oaths and vows which must not be kept, and so far are _quasi nulla_ as to the effecting of the thing vowed; but they are not simply null; for they have the effect of making the man a sinner and perjured. They are sinful vows, and therefore vows. A natural incapacity proveth it no vow at all; but if I am naturally capable, and only forbidden, (by God or man,) this maketh it not no vow, but a sinful vow, of which some must be kept and some must not. [Sidenote: Cases in which a vow must not be kept.] In these following cases a real vow is _quasi nullum_, or must not be kept.