Iul. Pacii Iuriscons. in L. frater à fratre D. de condictione indebiti, commentarius

In the power of the same father are Primus and Secundus: each has 20 in his peculium. Primus borrows ten from Secundus. Hence a natural obligation arises, for he owes ten.

A complex diagram displays the following logical structure:
Primus owes:

• To the father (this obligation is not considered, because it is primarily owed to the father).
• After the death of the father, the father succeeds into the obligation:
  • If Primus succeeds, the obligation is extinguished because the debtor succeeds the creditor.
  • If Secundus succeeds, the obligation remains as the creditor succeeds the debtor.
    The Father owes:
• To Secundus:
  • After the death of the father, the father succeeds into the obligation:
    • If Primus succeeds, the obligation remains as the debtor succeeds the debtor.
    • If Secundus succeeds, the obligation is extinguished because the creditor succeeds the debtor.
  • To himself: This obligation does not subsist, because one cannot be a debtor to oneself.

Therefore, Primus owes five to Secundus. But this obligation is removed if, in the division of the paternal inheritance, five from the peculium of Primus arrive at Secundus.