The Same, Digest, Book LIII. Where anyone promises to pay ten aurei if Titius should become Consul, even though the promisor should die while the condition is pending, he will leave his heir liable.

Ulpianus, Digest, Book LIII. For where what has been paid cannot be recovered, it is proper that a surety for this natural obligation should be received.

The Same, Digest, Book LIII. A surety cannot be rendered liable to a person to whom the principal debtor is not liable. Wherefore, if a slave owned in common by Titius and Sempronius is specifically stipulated to be given to Titius, and his surety should be asked, “Do you promise to give this to Titius, or Sempronius?” Titius, indeed, can demand it from the surety, but Sempronius appears to have been introduced for the sole purpose that payment might be made to him before issue is joined in the case, while Titius is not aware of the fact, or is unwilling that this should be done. 1A person who has promised to pay at a certain place is, to some extent, subjected to a more severe condition than if he had been simply interrogated, for he cannot make payment in any other place than that in which he agreed to pay, if the stipulator is unwilling for him to do so. Wherefore, if I interrogate the principal debtor absolutely, and I accept the surety with the addition of payment in a certain place, the surety will not be liable. 2Even if the principal debtor, while at Rome, should promise to make payment at Capua, and the security at Ephesus, the surety will not be liable any more than if the principal debtor had promised to pay under a condition, and the surety had agreed to do so on a certain day, or had promised absolutely. 3A surety can be accepted whenever any civil or natural obligation, which is applicable to him, exists. 4Natural obligations are not estimated solely by the fact that some action can be brought on account of them, but also where the money, once paid, cannot be recovered. For although natural debtors cannot strictly be said to be indebted, still they may be considered such, and those who receive money from them to have obtained that to which they were entitled. 5Where a stipulation has been entered into which is to take effect at a specified time, and a surety has been accepted under a condition, the rights of the latter will remain in suspense, so that, if the condition is complied with before the time prescribed, he will not be liable; but if the time and the condition should coincide, or if the. condition should be fulfilled after the specified time has elapsed, he will be liable. 6When a surety is accepted under the following terms, “Will you be responsible if the principal debtor does not pay the forty aurei which have been lent to him?” it is probable that the intention was that if the principal debtor did not pay when called upon, the surety would be liable; but if the principal debtor, before being notified to pay, should die, the surety will be liable, because, even in this case, it is true that the principal debtor did not make payment.