Adam Smith (1723-1790) is best known for his pioneering work of economics, The Wealth of Nations (1776). But the book that actually propelled him to fame was The Theory of Moral Sentiments, published in April in 1759.
It was a sensation, and it made Smith into hot intellectual property. That's because moralists had been struggling for centuries to work out the principles that made some actions morally good and others morally bad. To clerics, the answer was obvious: the word of God. And believers relied on the clerics' moral authority to guide them. Skeptics, on the other hand speculated about whether we had a sixth sense, a 'moral sense' that would guide us towards good. And so it went on.
Smith's breakthrough was to place our moral judgements as a matter of our deep psychology as social creatures. Human beings, he argued, have a natural 'sympathy' (today we would say 'empathy') for each other, particularly those nearest to them. That empathy enables them to understand how to adjust and moderate their behaviour in order to win the favour of others and preserve social harmony. It is the basis of moral judgements about behaviour, and the source of human virtue.
Writing exactly a century before Charles Darwin's The Origin of Species (1859), Smith was not sure why such beneficial social behaviour should prevail. He put it down to providence: today we would put it down to evolution.
The Theory of Moral Sentiments was an intellectual sensation, a best seller. Churchmen, of couse, did not like it very much. But it impressed Charles Townsend, a leading intellectual and senior member of the British government (roughly the equivalent of the Chancellor of the Exchequer today). He sought an introduction to Smith through their mutual friend, the philosopher David Hume (1711-1776). Townsend immediately hired Smith, on a salary of £300 a year for life, to be tutor to his stepson, the young Duke of Buccleuch. It was a small fortune. And it gave Smith the independence and experience to start writing the world for which he is best remembered today: The Wealth of Nations.