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

An example of a natural obligation is one that arises from a naked pact original: "nudo pacto" (Code 2.3.10), or that by which a slave is held (Digest 12.6.13, in the opening). The title of the Institutes on exceptions provides examples of a civil obligation, such as when someone has promised while compelled by fear or induced by fraud. A mixed one occurs in many places, as when someone has promised to one stipulating: for he is held both civilly (Institutes 4.13, § "de constituta") and naturally on account of consent (Digest 2.14.1, § 1).

It must be understood, furthermore, that between a father and a son who is under his power, as well as between two brothers established under the same power, not a civil, but only a natural obligation arises. I say that a civil obligation does not arise between father and son: because on account of the paternal power original: "patria potestas" there can be no judgment original: "iudicium" between them (Digest 5.1.4; 44.7.7). A civil obligation, however, as I have said above, is that from which an action and judgment is granted. As far as brothers are concerned, since an obligation is sought for the father through the son (Digest 44.7.1; Institutes 2.9, in the opening), it is necessary to extend to brothers what has been said about the father and son.

Neither should that be omitted: that he, who...