Great concepts in science do not just happen. They evolve, often along predictable lines. In an especially common scenario, an orderly set of related observations leads scientists to postulate an abstract entity or class whose existence somehow explains those relationships. Later on, the entity gets an unambiguous operational definition, through the application of some generally accepted experimental test. Frequently, in a third and final stage, the experimental test is replaced by a set of concrete features, typically physical, that more efficiently (if sometimes a little imperfectly) characterize the concept.