157Hence, we see that behind all attempts to represent the
 circulation of commodities as a source of surplus-value, there lurks a
 quid pro quo, a mixing up of use-value and exchange-value. For instance,
 Condillac says: “It is not true that on an exchange of commodities we give
 value for value. On the contrary, each of the two contracting parties in
 every case, gives a less for a greater value. ... If we really exchanged
 equal values, neither party could make a profit. And yet, they both gain,
 or ought to gain. Why? The value of a thing consists solely in its relation
 to our wants. What is more to the one is less to the other, and vice
 versâ. ... It is not to be assumed that we offer for sale articles
 required for our own consumption. ... We wish to part with a useless thing,
 in order to get one that we need; we want to give less for more. ... It
 was natural to think that, in an exchange, value was given for value, whenever
 each of the articles exchanged was of equal value with the same quantity
 of gold. ... But there is another point to be considered in our calculation.
 The question is, whether we both exchange something superfluous for something
 necessary.” [8] We see in this passage, how Condillac
 not only confuses use-value with exchange-value, but in a really childish
 manner assumes, that in a society, in which the production of commodities
 is well developed, each producer produces his own means of subsistence,
 and throws into circulation only the excess over his own requirements.
 [9] Still, Condillac’s argument is frequently used by
 modern economists, more especially when the point is to show, that the exchange
 of commodities in its developed form, commerce, is productive of surplus-value.
 For instance, “Commerce ... adds value to products, for the same products
 in the hands of consumers, are worth more than in the hands of producers,
 and it may strictly be considered an act of production.” [10]
 But commodities are not paid for twice over, once on account of their use-value,
 and again on account of their value. And though the use-value of a commodity
 is more serviceable to the buyer than to the seller, its money-form is
 more serviceable to the seller. Would he otherwise sell it? We might therefore
 just as well say that the buyer performs 158"strictly an act of
 production,” by converting stockings, for example, into money.