This book argues for the place of capacities within an grounds of meaning, not method. Yet it is questions of method that should concern the modern empiricist: can capacities be measured? Cartwright argues that they are measured if anything is. Stanford University's Gravity-Probe-B will measure capacities in a cryogenic dewar deep in space. More mundanely, we use probabilities to measure capacities, and the assumptions required to ensure that probabilities are a reliable instrument are investigated in the opening chapters of this book, where the early methods of econometrics set a model. The last chapter applies lessons about probabilities and capacities to quantum mechanics and the Bell inequalities. The central thesis throughout is that capacities not only can be admitted by empiricists, but indeed must be - otherwise the empirical methods of modern science will make no sense.