This book argues for the place of capacities within an empiricist philosophy. Hume wanted to expel them, but on 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'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.