Today, the Scottish thinker 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 on this day in 1759.

Moralists had been struggling to work out the principles that made some actions morally good and others morally bad. To churchmen, the answer was obvious: it was the word of God. Skeptics speculated about whether we had a sixth sense, a ‘moral sense’ that would guide us towards good.

Smith’s breakthrough was to identify our moral judgements as a matter of human beings’ deep psychology as social creatures. Human beings, he argued, have a natural ‘sympathy’ (today we would say ‘empathy’) for others. That enables them to understand how to moderate their behaviour and preserve 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 social behaviour should prevail. He put it down to providence: today we would put it down to evolution.

The book was an intellectual sensation. Churchmen, of course, did not like it very much, but it caught the eye of Charles Townsend, an intellectual and senior member of the British government, who was highly impressed and 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 fortune – and it gave Smith the independence and experience to start writing the work for which he is remembered today, The Wealth of Nations.