Studying systems whose components are concepts; for example, logical, numerical, linguistic, philosophical, ethical, and religious systems. Conceptual systems may differ in significant ways from concrete systems and in order to be considered within the framework of any general system theory, a system has to be defined more generally as a complex of interrelated entities (rather than of interacting entities). In order to study concrete systems, abstract systems with analogous relationships may be substituted such that the problem becomes a mathematical one. This process is usually known as the development of a model; the extent to which the abstract model agrees with the actual behaviour of the concrete, physical system is a measure of the applicability of the particular model to the situation in question.