The histories interpretation of quantum mechanics, as noted earlier, is probabilistic, and hence the processes of reasoning are those that scientists generally use for probabilistic models. This requires a method of assigning probabilities along with procedures for drawing inferences, usually in the form of conditional probabilities. In both these respects quantum theory resembles other applications of probabilistic reasoning. The main difference is the necessity in the quantum case of paying close attention to the problem of choosing a sample space (which then determines the event algebra) or framework, and then confining the logical reasoning process, from assumptions to conclusions, to a single framework. We have already encountered some applications of this single framework rule, and additional ones are discussed later, but at this point it is useful to summarize some of the basic principles involved, which can be conveniently placed under four headings: