Plato was an ancient Greek philosopher whose ideas on rationalism Haidt critiques while discussing The Primacy of Intuition and Emotion in Moral Judgment. He cites Plato’s dialogue, Timaeus, as evidence that Plato was overly reliant on the idea that reason could triumph over “passion” (emotion or instinct) and that reason alone could make sensible decisions. He also references Plato’s Republic in which Glaucon (Plato’s brother) challenges Socrates to prove that justice itself, and not just the reputation for justice, leads to happiness. Though Socrates is able to offer an explanation that Plato finds satisfactory, Haidt believes that Glaucon is right and that reputation matters more than right action.

Haidt refers frequently to the 18th-century philosopher David Hume, who argues that reason is the “slave” of the passions. Haidt believes Hume is right that emotion and intuitive knowledge are stronger than reason. However, Haidt objects to the term “slave,” as he does think that reason can employed if we can acknowledge the moral matrices in ourselves and others and use those foundations to change minds and opinions.