Thinking in terms of facts and rules is perhaps one of the most common ways of approaching problem definition and problem solving both in everyday life and under more formal circumstances. The best known set of rules, the Ten Commandments have been accompanying us since the times of Moses; the Decalogue proved to be simple but powerful, concise and universal. It is logically consistent and complete. There are also many other attempts to impose rule-based regulations in almost all areas of life, including professional work, education, medical services, taxes, etc. Some most typical examples may include various codes (e.g. legal or traffic code), regulations (especially military ones), and many systems of customary or informal rules. The universal nature of rule-based formulation of behavior or inference principles follows from the concept of rules being a simple and intuitive yet powerful concept of very high expressive power. Moreover, rules as such encode in fact functional aspects of behavior and can be used for modeling numerous phenomena.

There are two main types of rules depending on their origin: there are objective, physical rules defined for us by Nature and there are subjective, logical rules defined by man. Physical rules describe certain natural phenomena and behavior of various systems; they are known by observation and experience, sometimes they can be proved, having objective nature they are independent of our will, they are universal and normally cannot be changed. Logical rules are those defined by man; they are usually subjective, local, subject to change if necessary. Physical rules describe possible behavior — they can be used in domains such as modeling, analysis and prediction of system behavior. Logical rules are usually aimed at shaping the behavior of man, society or machine. In any case definition of logical rules must respect the necessity of taking into consideration physical rules which cannot be violated — physical rules are superior with respect to logical ones.