Some ideas seem to possess a disproportionate ability to lead to new insights, new discoveries, new ideas, and even entirely new ways of thinking. Such ideas are said to be fruitful. Looking across the history of science and mathematics, we see creative minds preoccupied with the search for ideas of this kind. More precious than truth, but far less plentiful, fruitful ideas provide those in pursuit of knowledge with a seemingly bottomless well of innovation from which to draw as they attempt to solve new problems and to refine solutions to old ones. Seasoned researchers have a nose for these ideas. They often know in an instant that some way of approaching a problem will eventually result in a solution to it and to a whole host of other problems, all of which suddenly seem related. In _Fruitfulness_, Chris Haufe explains how these ideas are detected and developed into large-scale frameworks for research. He argues for a philosophical perspective on scientific knowledge that places the search for fruitfulness at the heart of the scientific enterprise. This perspective demands a fundamental shift in our thinking about scientific theories, conceiving of them as metaphors to facilitate research instead of increasingly correct descriptions of nature.