But, nor can exchange create an increase in value. If A has a value equal to 10 hours of labour, and B equal to 20 hours of labour, 2A must exchange for 1B. A total of 40 hours of value exists, and this will not be changed, as a result of whether A and B are exchanged or not, or in what proportions A and B actually are exchanged. If 3A are given for 1 B, this simply means that the fall in the exchange value of A is simply compensated in a rise in the exchange value of B. The total value here is 30 hours of A, 20 hour of B = 50 hours. The value of A has fallen from 2:1, measured in B, but likewise the value of B has risen from 1:2 to 1:3. Yet the total amount of value remains the same, proportionally.
To be more precise, if only 2A, and 1B continued to exist, but they exchanged as 3:1, the 2A would obtain 0.66B in return, with 0.33B remaining in its owner's hands. That would not change the fact that 40 hours of value exists in total. The owner of 0.66B would now possess 13.33 hours of value, whilst the owner of the remaining 0.33B would possess 6.66 hours of value, and a further 20 hours of value, in the form of 2 units of A.
“So far as it gives A a greater value than it has before the exchange, it gives B a smaller value. A+B, therefore, has the same value after the exchange as it had before it.” (p 206)
Marx once again specifies his definition of “product” as set out in Capital I.
“First, if these things are “products”, they are from the start products of labour, not general elemental things provided by nature like air, etc....” (p 206)
On this basis, the most useful things – use values – may have no value, and consequently, no exchange value, because they are not, to begin with, products. They are not the product of labour. And, to the extent such useful things are products, and thereby possess value, they can only be devoid of exchange value, if they are produced for direct consumption.
By contrast, Ganilh writes,
““And the most useless products may have very great value, if exchange is favourable for them” (p. 104). “ (p 206)
But, as Marx points out, if such products are completely useless, why would anyone buy them? A precondition for any use value being a commodity is precisely that it is a use value, that someone must have a use for it, even if their belief that they have such a use is purely imaginary.
“And if he is not a fool, why should he pay more for them? Their dearness must therefore originate in some circumstance which in any case does not arise from their “uselessness”. Their “scarcity”, rarity? But Ganilh calls them “the most useless products”. As therefore they are products, why are they not produced in greater quantities, in spite of their great “exchange-value”? If before it was the buyer who was a fool, giving a lot of money for something that had neither a real nor an imaginary use-value for him, now it is the seller, who does not produce these trifles of great exchange-value instead of utilities of small value. That their exchange-value is great in spite of their small use-value (use-value determined by the natural needs of man), must therefore be due to some circumstance that originates not from Lord Exchange, but from the product itself. Its high exchange-value is therefore not the product of exchange, but only appears in exchange.” (p 206)
In short, if some commodity is really useless, no one will buy it, so, however much labour-time is expended on its production, it will not be socially necessary labour-time.