2. There are such things as _implicit contracts_, attached to the bearing of certain offices, whereby a man becomes his brother's keeper. The liability contracted is limited by the nature of the office: thus a physician is officially bound in justice as to his patient's pulse, but not officially as to his purse. Where there is no explicit contract, the duties which the subjects of a person's official care have towards him are not duties of commutative justice. Thus these _implicit contracts_ are not strictly contracts, as failing to carry a full reciprocity. 3. Contracts are either _consensual_ or _real_, according as they are either complete by the mere consent of the parties, or further require that something should change hands and pass from one to the other. What contracts are consensual, and what real, depends chiefly on positive law. No natural law can tell whether buying and selling, for instance, be a consensual or a real contract. The interest of this particular case is when the goods are lost in transmission: then whichever of the two parties at the time be determined to be the owner, apart from culpable negligence or contrary agreement of the sender, he bears the loss, on the principle, _res perit domino_. 4. Contracts are otherwise divided as _onerous_ and _gratuitous_. In an onerous contract either party renders some advantage in return for the advantage that he receives, as when Titius hires the horse of Caius. In a gratuitous contract all the advantage is on one side, as when Titius does not hire but borrows a horse. The Roman lawyers further distinguish contracts, somewhat humorously, into _contracts with names_ and _contracts without names_, or _nominate_ and _innominate_, as anatomists name a certain bone the _innominate bone_, and a certain artery the _innominate artery_. _Innominate contracts_ are reckoned four: _I give on the terms of your giving_, otherwise than as buying and selling,--to some forms of this there are English names, as _exchange_ and _barter_: _I do on the terms of your doing: I do on the terms of your giving: I give on the terms of your doing_. _Readings_.--De Lugo, _De Just. et Jure_, 22, nn. 1, 2, 5, 6, 9, 16, 17. For buying and selling and the frauds incident thereto, Paley, _Moral Philosophy_, bk. iii., p. 1, c. vii. SECTION V.--_Of Usury_. 1. We must distinguish _use value_ and _market value_. The use value of an article of property is the esteem which the owner has of it from every other point of view except as a thing to sell. Thus a man values his overcoat on a journey as a protection from cold and rain. A book is valued that was held in the dying hand of a parent. This is use value. The market value of an article is the estimate of society, fixing the rate of exchange between that and other articles, so much of one for so much of another, _e.g._, between mahogany and cedar wood, considered as things to sell.