4 A legacy may be given not only of things belonging to the testator or heir, but also of things belonging to a third person, the heir being bound by the will to buy and deliver them to the legatee, or to give him their value if the owner is unwilling to sell them. If the thing given be one of those of which private ownership is impossible, such, for instance, as the Campus Martius, a basilica, a church, or a thing devoted to public use, not even its value can be claimed, for the legacy is void. In saying that a thing belonging to a third person may be given as a legacy we must be understood to mean that this may be done if the deceased knew that it belonged to a third person, and not if he was ignorant of this: for perhaps he would never have given the legacy if he had known that the thing belonged neither to him nor to the heir, and there is a rescript of the Emperor Pius to this effect. It is also the better opinion that the plaintiff, that is the legatee, must prove that the deceased knew he was giving as a legacy a thing which was not his own, rather than that the heir must prove the contradictory: for the general rule of law is that the burden of proof lies on the plaintiff. 5 If the thing which a testator bequests is in pledge to a creditor, the heir is obliged to redeem it, subject to the same distinction as has been drawn with reference to a legacy of a thing not belonging to the testator; that is to say, the heir is bound to redeem only if the deceased knew the thing to be in pledge: and the Emperors Severus and Antoninus have decided this by rescript. If, however, the deceased expresses his intention that the legatee should redeem the thing himself, the heir is under no obligation to do it for him. 6 If a legacy is given of a thing belonging to another person, and the legatee becomes its owner during the testator's lifetime by purchase, he can obtain its value from the heir by action on the will: but if he gives no consideration for it, that is to say, gets it by way of gift or by some similar title, he cannot sue; for it is settled law that where a man has already got a thing, giving no consideration in return, he cannot get its value by a second title of the same kind. Accordingly, if a man is entitled to claim a thing under each of two distinct wills, it is material whether he gets the thing, or merely its value, under the earlier one: for if he gets the thing itself, he cannot sue under the second will, because he already has the thing without giving any consideration, whereas he has a good right of action if he has merely got its value. 7 A thing which does not yet exist, but will exist, may be validly bequeathed:--for instance, the produce of such and such land, or the child of such and such female slave.