you do not have heirs of your body let the land so given revert to such persons,  one or several, together or in succession.1 There is also a tacit condition phrased  affirmatively, as where it is said, If A is not heir I substitute B and C in turn,2  that is, I make them heirs;3 if one of them dies first the survivor succeeds. Some  conditions [are single], others double: [double], if you have no heirs of your body,  or if you have them and they fail, let the land given then revert to me and my  heirs. Others are double and phrased affirmatively, as If you have heirs and they  die within age let the land revert etc. Others are double and phrased partly in the  negative and partly in the affirmative, as If my son is insane, or if he is not and  dies before reaching puberty,4 I wish you to be my heir, and the like. A gift may  also be made subject to a modus with several conditions attached,5 as where I say,  I give you this thing that you do such a thing (or that you do not do it) and if  you do not do it (or if you do it) that the land revert to me,6 or I give that you  not do such act without my consent and if you do that I may re-enter and hold  myself there quit of you and your heirs.7 Hence, when the aforesaid conditions,  or the agreements and the documents in which they are contained, are denied in  court, it does not suffice if the instruments and conditions or agreements are proved  unless it is established by positive proof, or at least by a presumption effective  until the contrary is proved, that the condition was satisfied [or not satisfied,] for  if the instrument is proved it may still be that the condition was [not] satisfied.8  9[It is otherwise] if one gives subject to a modus which depends on the will and  power of another, for example, if one gives another the advowson of a church that  he establish a priory there, [which] cannot be accomplished without the assent  of the bishop or other ordinary; if they are unwilling to consent, that must not be  charged to the donee, and the gift will be good, especially if the donee has done everything  in his power to conclude the matter.
[a gift is made] because of a precedent act or causa.  A gift sometimes is made for a past causa, [sometimes] for one to follow: 10for a  past causa, as where I say, I give you this thing because you have served me well,  and here the gift is good though he has not served well, for the addition of a false  causa no more destroys a gift than it does a legacy.11 If it is made for a causa