XIII. There is a certain degree of equality, too, in beneficial or gratuitous acts, not indeed like that prevailing in contracts of exchange, but proceeding upon a supposition of the hardship, that any one should receive detriment from voluntary services, which he bestows. For which reason a voluntary agent ought to be indemnified for the expence or inconvenience, which he incurs, by undertaking the business of another. A borrower too is bound to repair a thing that has been damaged or destroyed. Because he is bound to the owner not only for the thing itself, by virtue of the property which he retains in it, but he owes a debt of gratitude also for the favour of the loan; unless it appears that the thing so lent would have perished, had it even remained in possession of the owner himself. In this case, the owner loses nothing by the loan. On the other hand, the depositary has received nothing but a trust. If the thing therefore is destroyed, he cannot be bound to restore what is no longer in existence, nor can he be required to make a recompence, where he has derived no advantage; for in taking the trust he did not receive a favour, but conferred one. In a pawn, the same as in a thing let out for hire, a middle way of deciding the obligation may be pursued, so that the person taking it is not answerable, like a borrower, for every accident, and yet he is obliged to use greater care, than a bare depositary, in keeping it safe. For though taking a pledge is a gratuitous acceptance, it is followed by some of the conditions of a contract. All these cases are conformable to the Roman law, though not originally derived from thence, but from natural equity. Rules, all of which may be found among other nations. And, among other works, we may refer to the third book and forty-second chapter of the GUIDE FOR DOUBTFUL CASES, written by Moses Maimonides, a Jewish writer. Upon the same principles the nature of all other contracts may be explained; but the leading features in those of certain descriptions seemed sufficient for a treatise like the present. XIV. The general demand for any thing, as Aristotle has clearly proved, constitutes the true measure of its value, which may be seen particularly from the practice prevailing among barbarous nations of exchanging one thing for another. But this is not the only standard: for the humours and caprice of mankind, which dictate and controul all regulations, give a nominal value to many superfluities. It was luxury, says Pliny, that first discovered the value of pearls, and Cicero has somewhere observed, that the worth of such things can only be estimated by the desires of men.