3 A stipulation by a slave belonging to joint owners enures to the benefit of all of them in proportion to the shares in which they own him, unless he stipulated at the bidding, or expressly in favour, of one of them only, in which case that one alone is benefited. Where a jointly owned slave stipulates for the transfer of property which cannot be acquired for one of his two masters, the contract enures to the benefit of the other only: for instance, where the stipulation is for the transfer of a thing which already belongs to one of them. TITLE XVIII. OF THE DIFFERENT KINDS OF STIPULATIONS Stipulations are either judicial, praetorian, conventional, or common: by the latter being meant those which are both praetorian and judicial. 1 Judicial stipulations are those which it is simply part of the judge's duty to require; for instance, security against fraud, or for the pursuit of a runaway slave, or (in default) for payment of his value. 2 Those are praetorian, which the praetor is bound to exact simply in virtue of his magisterial functions; for instance, security against apprehended damage, or for payment of legacies by an heir. Under praetorian stipulations we must include also those directed by the aedile, for these too are based upon jurisdiction. 3 Conventional stipulations are those which arise merely from the agreement of the parties, apart from any direction of a judge or of the praetor, and which one may almost say are of as many different kinds as there are conceivable objects to a contract. 4 Common stipulations may be exemplified by that by which a guardian gives security that his ward's property will not be squandered or misappropriated, which he is sometimes required to enter into by the praetor, and sometimes also by a judge when the matter cannot be managed in any other way; or, again, we might take the stipulation by which an agent promises that his acts shall be ratified by his principal. TITLE XIX. OF INVALID STIPULATIONS Anything, whether movable or immovable, which admits of private ownership, may be made the object of a stipulation; 1 but if a man stipulates for the delivery of a thing which either does not or cannot exist, such as Stichus, who is dead but whom he though alive, or an impossible creature, like a hippocentaur, the contract will be void.