Facts change all the time. Smoking has gone from doctor recommended to deadly. We used to think the Earth was the center of the universe and that the brontosaurus was a real dinosaur. In short, what we know about the world is constantly changing. Samuel Arbesman shows us how knowledge in most fields evolves systematically and predictably, and how this evolution unfolds in a fascinating way that can have a powerful impact on our lives. He takes us through a wide variety of fields, including those that change quickly, over the course of a few years, or over the span of centuries.