Recent years have witnessed the rapid emergence of a new scientific discipline, commonly known as system theory. The advent of this discipline was largely motivated by the acute need for a theoretical framework within which investigators of different fields of science and engineering could find a common language. The aim of this theory was to build up an arsenal of ideas and tools which would be of equal usefulness to specialists in a wide variety of fields, such as electrical engineering, mechanical engineering, physiology, and linguistics. This was achieved by viewing a system (which could be a living organism, an industrial establishment, a guided missile) not via its internal structure, but through the mathematical laws which govern its observable behavior. Employing this "black-box" approach, it was found that systems exhibiting completely dissimilar physical compositions could still be characterized in similar terms and analyzed through the same set of rules.