Summa summae S. Thomae, Tomus III

I say : Everyone is bound to restore at once, if he can ¹ .

Proof 1°. He who detains another's property against the reasonable wish of its owner sins in act ² . Yet it is not licit, no, not for the least time, to delay in sin, namely as to act and affect. Therefore. — Proof of the major. He who detains another's property against the reasonable wish of its owner deprives him of the use of his own thing, and thus does him an injury; for unjust detention of another's thing has the same character of harm as unjust acquisition. Therefore.

Proof 2°. Although the precept of making restitution according to its form is affirmative, it nevertheless implies within itself a negative, as the reason of its obligation, namely not to detain another's property ³ ; for therefore you are bound to restore because it is not licit to detain another's property. But negative precepts bind always and forever. Therefore.

You will ask 1° how that « at once » is to be understood.

I answer. If the speech concerns the purpose of restoring, it is understood at once mathematically, that is, at the very instant when you reflect that you are unjustly detaining another's property, you are bound not to will to detain it any longer, or to will to restore it, otherwise you sin against the negative precept by which detaining another's property is forbidden; just as he who conceived the purpose of stealing is bound at once mathematically to abandon it and not to will to steal, otherwise he sins against the precept : Thou shalt not steal.

But if the speech concerns external restitution, « at once » is understood morally, that is, at the first opportunity; because, since it is an external act, it necessarily requires an interval of time for its execution, which can be impeded by various obstacles or inconveniences, so that the owner can neither reasonably be unwilling nor ought he to be, if his thing is detained until it can be restored at a suitable place or time.

[Note that a creditor, unless this be imposed on him by pact or sentence of judge, is not bound to receive the debt in parts; because the debtor contracted the obligation of such a determined thing, not indeed of handing over or paying parts of this thing. So from natural and civil law ⁴ .

You will ask 2° whether damages ensuing from delay of restitution must be compensated.

I answer. A distinction must be made whether the obligation of restitution was born from contract or quasi-contract, or arisen from injury. If the former, either the cause of the delay is just, and damages are not to be compensated; because the delay is only their material cause, not formal. Or the cause is unjust, that is, the debtor is constituted culpably in delay, and damages are to be compensated, because they follow formally from the delay.

If the latter, all damages are to be compensated, even if the debtor be not culpably in delay, that is, though he be excused for just reason; because the delay follows from unjust acquisition or damaging, and therefore has become the formally unjust cause of the damage ⁵ .

This is certain concerning ordinary damages; but it is contested whether extraordinary damages also ought to be compensated, that is, those which follow from delay of restitution not necessarily but occasionally only.

It must beyond doubt be denied concerning those damages which do not follow necessarily, in the sense that they could easily have been forestalled by the owner of the thing ⁶ ; for these must be imputed to himself. It is also denied probably concerning damages which could not have been forestalled by the owner; for neither must they be imputed to the damager who, since he could not foresee them, nor consequently will them implicitly, was not their formally unjust cause.]

You will ask 3° whether he who unjustly detains another's property sins continually and at each moment.

I answer, more probably affirmative : because as long as he who unjustly detains another's property continually transgresses the implicitly negative precept of restoring, he continually injures his neighbor, impeding him from possession and use of his own thing or what is owed him; and this freely, because that transgression or injury continually hangs upon the free will not to restore. Therefore he sins continually.

You will ask 4° whether it be one sin alone continued, or several distinct in number.

I answer, that it is one sin alone continued, unless the first purpose be morally interrupted; because the first purpose virtually perseveres in voluntary continued detention, so that, while it is repeated, even a hundred times, it is reckoned the first being continued, and therefore one sin morally.

But the first purpose is reckoned morally interrupted when it is recalled, either formally through an opposite purpose, or virtually, when someone thinking about detaining further the other's property does not assent, but distracts his mind elsewhere, as if recalling the prior purpose.