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.
Le Commerce et le Gouvernement (1776), as quoted in Marx's Capital, Vol. I, Ch. 5.