“An exchange is a transaction in which the two contracting parties both gain. Whenever I make an exchange freely, and without constraint, it is because I desire the thing I receive more than that I give; and, on the contrary, he with whom I bargain desires what I offer more than that which he renders me. When I give my labour for wages it is because I esteem the wages more than what I should have been able to produce by labouring for myself; and he who pays me prizes more the services I render him than what he gives me in return.”