Self-reference as a principal indicator of complexity

Self-reference, the ability of a system to refer to itself, is a necessary condition for complexity, and can indeed form the basis for a definition of complexity. When a part of the system can refer to the whole of the system, there indeed have to be deeper complex structures and processes than those that only superficially appear. Examples of the centrality and essentialness of self-reference to complexity can be found in many fields of study, including mathematics, system of system architecture, and systems engineering processes.