Summa summae S. Thomae, Tomus III

1° : In commodato, the receiver is held for damage arising from light and lightest fault, but not from a fortuitous event. In precario, he is held from gross fault only. But in contracts which are for the utility of giver and receiver together, as in hiring, he is held from light fault.

So it is, not indeed from the nature of the thing, because, standing in the law of nature, no one from strict justice is bound to employ greater diligence in preserving the things of another than the owner himself would employ, but from the disposition of law both canonical and civil in many places.

And these laws seem to bind in the forum of conscience before the sentence of the judge ; because that obligation of restoring in these contracts seems not so much a punishment as a condition attached to them by the statutes of the laws. Nor should they be said to be founded upon a false presumption of fraud, but rather upon natural equity and honesty.

You will ask whether he who sends back by a messenger a thing hired, or lent for use, or deposited, is held to restitution or payment if the thing perish through the fault of the messenger.

I answer with Silvius, with a distinction. If the messenger was commonly held to be trustworthy, he is not bound, because he is not at fault. If the messenger was not commonly held to be trustworthy, he is bound, because he is at fault.

Nor is it similar concerning him who would send back a thing loaned mutuum ; because the thing loaned mutuum belongs to the borrower so long as it has not reached the lender¹ . Whence, although it perish without the fault of the borrower, it perishes to himself as owner. But a thing lent for use² , hired, or deposited, belongs to the lender for use, the hirer, or the depositor, and not to the bailee for use, the lessee, or the depositary. Whence, if it perish without his fault, he is held to nothing, and the thing perishes to its owner.

I say 2° : Those who act from quasi-contract, or from office, such as tutors, judges, advocates, physicians, surgeons, etc., do not seem to be held for damage arising from any juridical fault whatever, unless it be at the same time theological : because either there are no laws which clearly decree this concerning those offices, as we said holds for true and open contracts; or, if there are, they do not seem received in practice with respect to the forum of conscience; for this the common sense of men persuades. For there is no one, or scarcely anyone, who thinks that a physician, surgeon, advocate, tutor, and their like, are held in conscience and before God to restitution, if they have not sinned in conscience and before God. And indeed, if matters stood otherwise, who, though most vigilant and most expert, would wish to discharge these offices, since it often happens that some light or lightest juridical fault creeps in upon the most vigilant and most expert through inadvertence or natural forgetfulness, or some other case, on account of which they would be held to restore, although they were before God without fault?

Moreover, I confess that those who render service under office for pay are bound by the strict law of nature to employ greater diligence than those who serve gratuitously out of charity, and if they voluntarily omit it, they sin before God and are held to restitution.