“Money is a crystal formed of necessity in the course of the exchanges, whereby different products of labour are practically equated to one another and thus by practice converted into commodities.”

Source: Das Kapital (Buch II) (1893), Vol. II, Ch. X, p. 211.

Source: The Wealth of Nations (1776), Book I, Chapter V.
Context: Every man is rich or poor according to the degree in which he can afford to enjoy the necessaries, conveniences, and amusements of human life. But after the division of labour has once thoroughly taken place, it is but a very small part of these with which a man's own labour can supply him. The far greater part of them he must derive from the labour of other people, and he must be rich or poor according to the quantity of that labour which he can command, or which he can afford to purchase. The value of any commodity, therefore, to the person who possesses it, and who means not to use or consume it himself, but to exchange it for other commodities, is equal to the quantity of labour which it enables him to purchase or command. Labour, therefore, is the real measure of the exchangeable value of all commodities.

Pages 713–714.
"The Marxian Theory of Value: Das Kapital: A Criticism" (1884)
Context: It is true also that Marx elsewhere virtually defines value so as to make it essentially dependent upon human labour (p. 81 [43a]). But for all that his analysis is based on the bare fact of exchangeability. This fact alone establishes Verschiedenkeit and Ghichheit, heterogeneity and homogeneity. Any two things which normally exchange for each other, whether products of labour or not, whether they have, or have not, what we choose to call value, must have that "common something" in virtue of which things exchange and can be equated with each other; and all legitimate inferences as to wares which are drawn from the bare fact of exchange must be equally legitimate when applied to other exchangeable things. Now the "common something," which all exchangeable things contain, is neither more nor less than abstract utility, i. e. power of satisfying human desires. The exchanged articles differ from each other in the specific desires which they satisfy, they resemble each other in the degree of satisfaction which they confer. The Verschiedenheit is qualitative, the Gleichheit is quantitative.It cannot be urged that there is no common measure to which we can reduce the satisfaction derived from such different articles as Bibles and brandy, for instance (to take an illustration suggested by Marx), for as a matter of fact we are all of us making such reductions every day. If I am willing to give the same sum of money for a family Bible and for a dozen of brandy, it is because I have reduced the respective satisfactions their possession will afford me to a common measure, and have found them equivalent. In economic phrase, the two things have equal abstract utility for me. In popular (and highly significant) phrase, each of the two things is worth as much to me as the other.Marx is, therefore, wrong in saying that when we pass from that in which the exchangeable wares differ (value in use) to that in which they are identical (value in exchange), we must put their utility out of consideration, leaving only jellies of abstract labour. What we really have to do is to put out of consideration the concrete and specific qualitative utilities in which they differ, leaving only the abstract and general quantitative utility in which they are identical.This formula applies to all exchangeable commodities, whether producible in indefinite quantities, like family Bibles and brandy, or strictly limited in quantity, like the "Raphaels," one of which has just been purchased for the nation. The equation which always holds in the case of a normal exchange is an equation not of labour, but of abstract utility, significantly called worth. … A coat is made specifically useful by the tailor's work, but it is specifically useful (has a value in use) because it protects us. In the same way, it is made valuable by abstractly useful work, but it is valuable because it has abstract utility.

Source: Democracy Realizedː The Progressive Alternative (1998), p. 32

Source: Interregional and international trade. (1933), p. 30.

Source: Money and Credit in Capitalist Economies, 1990, p. 10; Cited in Howard Stein. "Theories of institutions and economic reform in Africa." World Development 22.12 (1994): 1833-1849.

Source: Europe and the People Without History, 1982, Chapter 11, The Movement of Commodities, p. 314.
Context: Where Adam Smith and David Ricardo had envisaged a growing worldwide division of labor, they had thought that each country would freely select the commodities it was most qualified to produce, and that each would exchange its optimal commodity for the optimal commodity of others. Thus in Ricardo's example, Britain would send Portugal its textiles, while Britons would consume Portuguese wines in turn. What his vision of free commodity exchange omits are the constraints that governed the selection of particular commodities, and the political and military sanctions used to ensure the continuation of quiet asymmetrical exchanges that benefited one party while diminishing the assets of another.

Source: The Limits To Capital (2006 VERSO Edition), Chapter 9, Money, Credit And Finance, p. 253

Lecture II, "Circumscription of the Topic"
1900s, The Varieties of Religious Experience (1902)