"Observers are men, animals, or machines able to learn about their environment and impelled to reduce their uncertainty about the events which occur in it, by dint of learning... [We] shall examine human observers who, because we have an inside understanding of their observational process, belong to a special category. For the moment, we shall not bother with HOW an observer learns, but will concentrate upon WHAT he learns about, i.e. what becomes more certain."