"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 versa. … 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."