Marx starts with the thing a shopper picks up, because in a society ruled by capital, wealth shows itself as "an immense accumulation of commodities". A commodity is first of all a useful object, and its usefulness is its use-value. Iron, corn, linen and diamonds all have one, and it only becomes real when someone uses or consumes the thing. But in the market the same object also has an exchange-value, and that looks accidental and always changing. A quarter of wheat swaps for so much blacking, so much silk or so much gold, in many different proportions. If all of those are worth one quarter of wheat, they must be equal to each other, so something common must sit behind every one of them. Marx takes corn and iron to press the point. One quarter of corn equals some quantity of iron, so both are equal to a third thing that is neither. That third thing cannot be weight, colour or any natural property, because exchange ignores use-value completely. An hundred pounds' worth of lead is as good as an hundred pounds' worth of silver. Strip away the table, the house and the yarn, and the one thing left is that each was made by human labour. Not the labour of the joiner or the mason, but human labour in the abstract, a "congelation" of the same stuff. Seen as crystals of that substance, the products are values.
Value needs a measure, and Marx gives it as labour-time, but not any labour-time. An idle or clumsy worker does not make a more valuable coat by taking longer. Only the labour socially necessary counts: the time needed under normal conditions, with the average skill and intensity of the day. His proof is the power-loom. When it came to England it cut the labour needed to weave yarn into cloth by about half. The hand-loom weavers still took the same hours as before, but an hour of their work now counted as only half an hour of social labour, and their cloth fell to half its value. Value also moves with how productive labour is. Eight bushels of corn may hold the same labour in a good season as four in a bad one. A rich mine gives more metal for the same work than a poor mine. Diamonds are costly because they take so much labour to find, and if someone could turn carbon into diamonds cheaply, they might fall below the value of bricks. Marx then draws the edges of the idea. Air, virgin soil and natural meadows are useful but have no value, because no labour went into them. A man who feeds himself from his own work makes use-values but not commodities. Even corn made for someone else is not a commodity if it is just owed. The medieval peasant grew quit-rent corn for his lord and tithe corn for his parson, and neither crop was exchanged. A product becomes a commodity only when it is made for others and passes to them through exchange. That brings Marx to the labour itself, which he says has the same double nature as the thing it makes.
Take a coat and ten yards of linen, with the coat worth twice the linen. The coat comes from tailoring and the linen from weaving. These are different kinds of useful labour, and they must be different, because coats are not swapped for coats. That difference spreads into a whole social division of labour. Marx adds that division of labour does not need commodities. A primitive Indian community has it, and so does every factory, where the work is split by plan and the workers do not trade their products with each other. Only products made by separate, private producers meet as commodities. Useful labour is also a condition of human life in every society, and it is not the only source of wealth, since, in William Petty's words, labour is the father of wealth and the earth is its mother. But in the value of the coat and the linen, tailoring and weaving lose their difference. Each is just an expenditure of human brains, nerves and muscles. In a capitalist society, human labour is supplied as tailoring at one time and as weaving at another, as demand changes. Skilled labour counts as simple labour multiplied, and a social process behind the backs of the producers fixes the rates. Productiveness then splits the two sides apart. Two coats clothe two men and one coat clothes one, so more coats mean more wealth. But an hour of labour yields the same value whatever its productiveness, so a change that lets the tailor make more coats in the same hours cuts the value of each coat. Wealth can grow while value falls.
