Michael Lewis tells the story of Daniel Kahneman and Amos Tversky's partnership, which revolutionized how we understand human judgment and decision-making. Kahneman and Tversky were Israeli psychologists whose collaboration produced findings that upended economics, medicine, law, and public policy. Lewis traces their relationship from their meeting in 1969 through decades of intellectual partnership and eventual estrangement. Their work demonstrated that human judgment relies on mental shortcuts that produce systematic errors, founding the field of behavioral economics. Lewis, known for narrative nonfiction that explains complex subjects through character and story, brings these ideas to life through the personalities of their creators. The collaboration was unusually intimate, with the two finishing each other's sentences and spending years developing ideas together. Lewis shows how their different temperaments—Kahneman's gloom, Tversky's confidence—both drove the partnership and eventually strained it. The book is a biography, a love story, and an introduction to ideas that have transformed multiple fields.