“If everything is scheduled in terms of one selected commodity it is indefinitely easier than it would otherwise be to realise the terms on which alternatives are open to us; and if any man defines his marginal estimate of anything he possesses in terms of this standard commodity any other member of the community will at once know whether or not it stands higher on his own scale than on the other's, and therefore whether or not the conditions for a mutually advantageous exchange exist.”

Pages 135–138.
The Common Sense of Political Economy (1910), Systematic and Constructive (Book I), "Money and Exchange" (ch. 4)
Context: In a great and complex industrial society direct reciprocity of services will not be the rule. I, Robinson, may (as before) want to have my old potatoes preserved and may not have the conveniences and capacities which give me exceptional qualifications for the task; whereas you, Jones, may have what I want; but I may have no relatively superior opportunities for rendering any corresponding service to you. I may, however, know Brown, who is good at growing the new potatoes you like, but has no special taste for them; and he may want nets mending or making, to put over his fruit-trees. I may, through physical constitution, acquired skill, or any other circumstance, be relatively better qualified, or in a better position, for making or mending nets than for either growing new potatoes or preserving old ones, and so I may do netting for Brown and get new potatoes, not because I want them myself, but because I know you want them, and I can barter them with you for the old potatoes you have preserved. Here I make nets which (relatively to the trouble of making them) I do not want, and I give them to Brown for new potatoes that I do not (relatively) want either, because I know that you who want new potatoes will give old potatoes for them, to which old potatoes I do attach a value that compensates me for the work I put into the nets. Or if you know about Brown and his tastes, you may give me old potatoes for my nets, not because you want nets, but because you want new potatoes and know that Brown, who has them, will give them to you in exchange for nets. Thus each is making what some one else wants in order to get what he wants himself. Further, if it is a fruit-growing and market-gardening country, you, without knowing any specific Brown who has new potatoes and wants nets, and without indeed there being any such person at all, may be willing to give me old potatoes for nets because you are pretty certain of finding a Smith somewhere who has new potatoes and will give them to you on suitable terms in exchange for nets, not because he wants nets either, but because he, in his turn, will by-and-by want cherries, which he does not grow, but expects to be able to get in exchange for nets from Williams. We need not carry the illustration any further to see that any article which is well known to be valued by a large and easily accessible class of persons may be taken habitually in exchange for valued commodities, although those who take it do not want it for their own use, and it does not, on its own merits, occupy such a place on their relative scale as would justify the exchange. All that is necessary is that there should be a confident expectation of finding some one on whose relative scale it does take such a place. The derivative value that such an article will possess in the mind of a man who has no direct use for it will depend on the direct value which it is conjectured to have in the mind of some accessible though not definitely identified individual or individuals. If there is some article of very generally recognised value which actually takes its place, as directly significant, on the scales of a great number of people, it may come to be generally accepted, without any special calculation or consideration, by people who are not thinking of any use they may have for it themselves, but are aware that it occupies a sufficiently high relative place on the scales of others to recoup them for what they give in exchange for it. As soon as this custom begins to be well established it will automatically extend and confirm itself, and the commodity in question will become a "currency" or "medium of exchange," the special characteristic of a medium of exchange being that it is accepted by a man who does not want it, or does not want it as much as what he gives for it, in order that he may exchange it for something he wants more. If I have some potatoes and should prefer some cherries, and give my potatoes for some nets, which I do not want as much, because I know that some one else has the cherries and will prefer nets to them, then the nets are a "medium" by the intervention of which I can, at two removes, exchange my potatoes for the cherries, though I cannot find any one who has the cherries and will give them to me for the potatoes. Postage stamps often serve as a medium of exchange, because a large and easily accessible class of persons are constantly wanting the services that the stamps will command. Tram tickets, when issued in books, might and to a limited extent do serve as a medium of exchange in the same manner. Cook's coupons might easily pass as a medium of exchange amongst travellers on the Continent; and if the railway companies issued their dividends in the shape of claims for such and such a mileage of travelling on their lines the certificates would be readily accepted in exchange by people who had no intention of travelling themselves, if they could make sure of finding people who did want to travel and would give them valuables in exchange for the claims. It is a matter of common knowledge that cattle still perform this function of a medium of exchange in South Africa, and books tell us that furs were long used as currency by the traders on Hudson Bay, and tobacco by the planters in Virginia.Concurrently with these developments, or perhaps in advance of them, the custom will grow up of estimating the marginal significance of things in terms of the generally accepted article even when the article does not pass from hand to hand in exchanges. There is more evidence in the Homeric poems of the valuation of female slaves, of tripods, or of gold or brass armour, in terms of so many head of cattle, than there is of any direct transfer of cattle in payment for other goods. The convenience of such a standardising of values is obvious. If everything is scheduled in terms of one selected commodity it is indefinitely easier than it would otherwise be to realise the terms on which alternatives are open to us; and if any man defines his marginal estimate of anything he possesses in terms of this standard commodity any other member of the community will at once know whether or not it stands higher on his own scale than on the other's, and therefore whether or not the conditions for a mutually advantageous exchange exist.In England the functions of a standardising commodity and of a medium of exchange are both alike performed by gold. Gold is applied to a vast number of purposes in the arts and sciences, and were it more abundant it would replace other metals in many more. Consequently a great number of easily accessible persons actually give a relatively high place to gold on their scales of preference, in virtue of its direct significance to them. It is established by custom (and, so far as that is possible, by law) as the universally accepted commodity; and at the same time it is used as the common measure in terms of which our estimates of all exchangeable things may be stated.