If you've ever taken an economics class, you were probably taught that people are rational. But about 50 years ago, the psychologists Daniel Kahneman and Amos Tversky began to chip away at this basic assumption. In doing so, they transformed our understanding of human behavior. This week, we remember Kahneman, who recently died at the age of 90, by revisiting our 2018 and 2021 conversations with him.