Adam Smith, The Invisible Hand, the exchange of goods between two parties

"It is not from the benevolence of the butcher, the brewer, or the baker, that we expect or dinner, but from their regard to their own interest.  We address ourselves, not to their humanity but to their self-love, and never talk to them of our own necessities but of their advantages.  Nobody but a beggar chooses to depend chiefly upon the benevolence of his fellow citizens."
-Adam Smith, The Wealth of Nations, volume I, page 16