Adam Smith published The Theory of Moral Sentiments in 1759.

It served as the foundation for Smith's later writings, such as The Wealth of Nations (1776), Essays on Philosophical Subjects (1795), and Lectures on Justice, Police, Revenue, and Arms (1763), by providing the ethical, philosophical, psychological, and methodological principles (first published in 1896). As the principle of sympathy replaces that organ, Smith broke with the "moral sense" tradition of Shaftesbury, Hutcheson, and Hume. Smith referred to the experience of these moral impulses as "sympathy." It was the sentiment shared by other people's passions. It operated on the principle of mirroring, whereby a viewer imagined reconstructing the experience of the subject they were watching:

Since we are unable to directly experience the emotions of other people, the only way we can imagine how we would feel in a similar circumstance is to imagine how we would feel. Even though our brother is in jail, as long as we are comfortable, our senses will never make us aware of the suffering he is going through. They never carried us outside of our own person and will never be able to, thus the only way we can have any idea of what his sensations are is through our imagination. Additionally, that teacher is unable to assist us in any other way than by illustrating what our position would be if we were in his position.