2 and labour, is not a matter of mere speculation, but may sometimes be of considerable use in practice. The same real price is always of the same value; but on account of the variations in the value of gold and silver, the same nominal price is sometimes of very different values. At the same time and place the real and the nominal price of all com- modities are exactly in proportion to one another. The more or less money you get for any commodity, in the London market, for example, the more or less labour it will at that time and place enable you to purchase or command. At the same time and place, therefore, money is the exact measure of the real exchangeable value of all commodities. It is so, however, at the same time and place only. The Wealth of Nations, Modern Library, 1937, pp. 33, 37. Alfred Marshall: .when considering costs from the social point of view, when inquiring whether the cost of attaining a given result is increasing or diminishing with changing economic conditions, then we are concerned with the real costs of efforts of various qualities, and with the real cost of waiting. If the purchasing power of money, in terms of effort has remained about constant, and if the rate of remuneration for waiting has remained about constant, then the money measure of costs corresponds to the real costs: but such a correspondence is never to be assumed lightly. Principles of Economics, Macmillan, 8th Ed., 1920, p. 350. John Stuart Mill: Exchange value requires to be distinguished from Price. The words Value and Price were used as synonymous by the early political economists, and are not always discriminated even by Ricardo. But the most accurate modern writers, to avoid the wasteful expenditure of two good scientific terms on a single idea, have employed Price to express the value of a thing in relation to money; the quantity of money for which it will exchange. By the price of a thing, therefore, we shall henceforth understand its value in money; by the value, or exchange value of a thing, its general power of purchasing; the command which its possession gives over purchaseable com- modities in general. The distinction between Value and Price, as we have now defined them, is so obvious, as scarcely to seem in need of any illustration. But in political economy the greatest errors arise from overlooking the most ob- vious truths. Simple as this distinction is, it has consequences with which a reader unacquainted with the subject would do well to begin early by making himself thoroughly familiar. The following is one of the prin- cipal. There is such a thing as a general rise of prices. A11 commodities may rise in their money price. But there cannot be a general rise of values. It is a contradiction in terms. A can only rise in value by exchanging for a greater quantity of B and C; in which case these must exchange for a smaller quantity of A. A11 things cannot rise relatively to one another. If one-half of the commodities in the market rise in exchange value, the very terms imply a fall of the other half; and reciprocally, the fall implies a rise. Things which are exchanged for one another can no more all fall, or