From pages 134 and 135 of The Righteous Mind: Why Good People Are Divided by Politics and Religion, by Jonathan Haidt:

The Birth of Moral Science

Nowadays, secular people often see the Enlightenment as a battle between two mortal enemies: on one side was science, with its principal weapon, reason, and on the other was religion, with its ancient shield of superstition. Reason defeated superstition, light replaced darkness. But when David Hume was alive, he was fighting a three-way battle. Enlightenment thinkers were united in rejecting divine revelation as the source of moral knowledge, but they were divided as to whether morality transcended human nature-that is, it emerged from the very nature of rationality and could therefore be deduced by reasoning, as Plato believed-or whether morality was a part of human nature, like language or taste, which had to be studied by observation.6 Given Hume’s concerns about the limits of reasoning, he believed that philosophers who tried to reason their way to moral truth without looking at human nature were no better than theologians who thoughtthey could find moral truth revealed in sacred texts. Both were transcendentalists.7

Hume’s work on morality was the quintessential Enlightenment project: an exploration of an area previously owned by religion, using the methods and attitudes of the new natural sciences. His first great work, A Treatise of Human Nature, had this subtitle: Being an Attempt to Introduce the Experimental Method of Reasoning into Moral Subjects. Hume believed that “moral science” had to begin with careful inquiry into what humans are really like. And when he examined human nature-in history, in political affairs, and among his fellow philosophers-he saw that “sentiment” (intuition) is the driving force of our moral lives, whereas reasoning is biased and impotent, fit primarily to be a servant of the passions.8 He also saw a diversity of virtues, and he rejected attempts by some of his contemporaries to reduce all of morality to a single virtue such as kindness, or to do away with virtues and replace them with a few moral laws.

Because he thought that morality was based in a variety of sentiments, which give us pleasure when we encounter virtue and displeasure when we encounter vice, Hume often relied upon sensory analogies, and particularly the taste analogy:

Morality is nothing in the abstract Nature of Things, but is entirely relative to the Sentiment or mental Taste. of each particular Being; in the same manner as the Distinctions of sweet and bitter, hot and cold, arise from the particular feeling of each Sense or Organ. Moral Perceptions therefore, ought not to be class’d with the Operations of the Understanding, but with the Tastes or Sentiments.9

Moral judgment is a kind of perception, and moral science should begin with a careful study of the moral taste receptors. You can’t possibly deduce the list of five taste receptors by pure reasoning, nor should you search for it in scripture. There’s nothing transcendental about them. You’ve got to examine tongues.

Hume got it right. When he died in 1776, he and other sentimentalists’O had laid a superb foundation for “moral science,” one that has, in my view, been largely vindicated by modem research.ll You would think, then, that in the decades after his death, the moral sciences progressed rapidly. But you would be wrong. In the decades after Hume’s death the rationalists claimed victory over religion and took the moral sciences off on a two-hundred-year tangent.