2. Answering to this twofold value is a twofold exchange, _private exchange_, which regards use value; and _commercial exchange_, which is founded on market value. If I part with my watch to a sailor for carrying me across an arm of the sea where there is no public ferry, that is private exchange. If I pay the ordinary fare where there is a public ferry, that is commercial exchange. 3. Private exchange begins in the need of at least one of the contracting parties. It is an act of charity in the other party to accommodate him by offering the thing needed. If the offer is made otherwise than as a gift, and is accepted, he who avails himself of it is bound in justice to see that the afforder of the accommodation is compensated for the loss that he suffers in affording it. Thus far the recipient is bound in justice, and no further in that virtue. However wholesome or profitable the thing be to him that gets it, the supplier cannot charge for that but only for the loss that he himself suffers, or the gain that he foregoes, in handing the thing over, or the pains that he takes, or the hardship that he endures, or the risk that he runs, in rendering the service desired. If all the labour to be undergone, or damage incurred, or risk encountered, by the sailor who goes about by private bargain to be my ferryman, is fairly met by the remuneration of a thirty-shilling watch, he has no right to stipulate for any more, not though the passage that he gives me sets me on the way to a throne. The peculiar advantage that I have in prospect does not come out of him, but out of myself. He must not pretend to sell what is not his, what attaches, not to him, but to me. He can only sell his own loss, risk, pains and labour. At the same time, if I have any gentlemanly or generous feeling in me, I shall be forward to bestow extra remuneration on one who has rendered me so timely a service: but this is matter of my gratitude, not of his right and claim in justice. Gratitude must not be put into the bill. And this much of private exchange. 4. Commercial exchange is conducted according to market value. Apart from dire necessity--and one in dire necessity is not fit to enter into commercial exchanges--the rule is, that a seller may always ask the market value of his article, however much that may be above what the thing cost him, or the use value which it bears to him. Thus, if one finds in his garden a rare Roman coin--so far as his tastes go, a paltry bit of metal--he may sell it for whatever price numismatists will offer: whereas, if there were no market for coins, but only one individual who doted on such things, the finder could make no profit out of that individual, the coin having neither market value with the community, nor use value in the eyes of the finder.