Summa summae S. Thomae, Tomus III

: Two titles are commonly assigned as inducing the obligation of restitution, namely a thing received and an unjust acquisition¹ .

1° By reason of the thing received he is bound to restore² , who has the thing of another in his hands without his fault, as a thing deposited, lent for use, pledged, found, etc.; because, since he has what is not his own but another's, what he has beyond what is his own ought to be withdrawn from him and given to him who lacks it, according to the form of commutative justice, so that equality may come about.

Moreover, although in that case there has been no unjust acquisition, nor is there an unjust detention, it would nevertheless be unjust if the thing were not returned in its time.

Under the name of the thing, however, is understood not only the thing existing in itself, but its value; likewise existing fruits, and all that by means of which whoever holds the thing has been made richer by it.

2° By reason of an unjust acquisition he is bound to restore³ , who has taken away the thing of another by theft, robbery, or some other unjust way, even if it does not remain with him. Likewise he who has unjustly damaged another without the reception of any thing; for under unjust acquisition is here understood unjust damaging.

The reason is that, although this man does not have more than he ought to have, the other nevertheless has less than he ought to have through his unjust action. Hence he violated justice toward him, which he is bound to repair by restitution.

But if the thing unjustly taken remains with him in itself or in its value, or if he has become richer thereby, then he is bound to restore under a twofold title, namely of the thing received and of the unjust acquisition. Furthermore, these two titles are not rarely joined together, although either one alone suffices.

Note 1°. Between these two titles there intervenes a great difference, to be diligently noted for practice : namely that he who is bound to restore by reason of an unjust acquisition remains under the obligation of restoring even if the thing owed perish without his fault; because the reason or title of the obligation always remains, namely the violation of justice, until it be repaired by restitution. Nor is he bound to restore only the thing or its value, but also all the fruits, even those consumed; likewise all damages ensuing thence and profits ceasing, in favor of him whom he unjustly damaged, because he is culpably the cause of them. But he who is bound to restore only by reason of the thing received is not bound to restore if the thing perish without his fault, as, v. g., in a deposit; because then the reason or title of the obligation ceases, which is to have the thing of another. Nor is he bound either to restore the fruits, if they are consumed and he has not been made richer thereby, nor the damages ensuing or profits ceasing; because he is not their cause. — Of these things more fully again in what follows, while we treat of the possessor in good faith and in bad faith.

Note 2°. A third title inducing the obligation of restitution could be assigned, namely contract; for by the force of contract alone one can be obliged to the restitution of a thing which he no longer has, nor received unjustly. But because from a contract there seems to arise rather an obligation of payment than of restitution, and the two former titles alone are commonly assigned as known to all, that we may speak as all speak, and lest confusion be engendered, we too assign only these two titles. Nevertheless we shall speak below of the obligation of paying or restoring from contract.