He wrote an earlier book, in 1759, called "The Theory of Moral Sentiments". He thought that empathy was very important in ethics. Empathy is when you see how someone else is feeling, imagine what it feels like, and then end up feeling the same way: for Smith, empathy is like "putting yourself in someone else's shoes". For example, empathy makes us feel happy to see someone else who is happy, or feel sorry for someone who is sad. Empathy may even make "us" feel pain when we see someone in pain (as if "we" had been hurt or affected). Both Smith and his friend David Hume, who was another Scottish philosopher, wrote about the importance of sympathy in ethics.