This book is an investigation of physical probability, the kind of probability involved in irreducibly random processes. In a random process, later events seem to be loosely attached to earlier ones; some tight relationship, which is present in deterministic systems, is missing. This latter relationship is sometimes held to be the relation of cause and effect, and thus random events are not caused by what preceded them. Richard Johns, however, adopts the stance that random events are fully caused and lack only determination by their causes. According to this causal theory of chance, the physical chance of an event is the degree to which the event is determined by its causes.