Systems thinking is a way of making sense of the complexity of the world by looking at it in terms of wholes and relationships rather than by splitting it down into its parts.^[1][2] It has been used as a way of exploring and developing effective action in complex contexts,^[3] enabling systems change.^[4][5] Systems thinking draws on and contributes to systems theory and the system sciences.^[6]

• "...What is a system? A system is a set of things ... interconnected in such a way that they produce their own pattern of behavior over time. ... But the system’s response to these forces is characteristic of itself, and that response is seldom simple in the real world".—Donella Meadows^[19]: 2

  • Subsystems serve as part of a larger system, but each comprises a system in its own right. Each frequently can be described reductively, with properties obeying its own laws, such as Newton's System of the World, in which entire planets, stars, and their satellites can be treated, sometimes in a scientific way as dynamical systems, entirely mathematically, as demonstrated by Johannes Kepler's equation (1619) for the orbit of Mars before Newton's Principia appeared in 1687.
  • Black boxes are subsystems whose operation can be characterized by their inputs and outputs, without regard to further detail.^{[19]: 87–88 [29]}
• "system: (1) An integrated whole even though composed of diverse, interacting, specialized structures and subjunctions".^[17]: 582
  • Political systems were recognized as early as the millennia before the common era.^[30][31]
  • Biological systems were recognized in Aristotle's lagoon ca. 350 BCE.^[32][33]
  • Economic systems were recognized by 1776.^[34]
  • Social systems were recognized by the 19th and 20th centuries of the common era.^[35][36]
  • Radar systems were developed in World War II in subsystem fashion; they were made up of transmitter, receiver, power supply, and signal processing subsystems, to defend against airborne attacks.^[37]
  • Dynamical systems of ordinary differential equations were shown to exhibit stable behavior given a suitable Lyapunov control function by Aleksandr Lyapunov in 1892.^[38]
  • Thermodynamic systems were treated as early as the eighteenth century, in which it was discovered that heat could be created without limit, but that for closed systems, laws of thermodynamics could be formulated.^[39] Ilya Prigogine (1980) has identified situations in which systems far from equilibrium can exhibit stable behavior;^[40] once a Lyapunov function has been identified, future and past can be distinguished, and scientific activity can begin.^{[39]: 212–213}

Frameworks and methodologies for systems thinking include:

• Critical systems heuristics:^[44] in particular, there can be twelve boundary categories for the systems when organizing one's thinking and actions.^[44]
• Critical systems thinking, including the E P I C approach.
• Ontology engineering of representation, formal naming and definition of categories, and the properties and the relations between concepts, data, and entities.
• Soft systems methodology, including the CATWOE approach and rich pictures.
• Systemic design, for example using the double diamond approach.
• System dynamics of stocks, flows, and internal feedback loops.
• Viable system model: uses 5 subsystems 1 2 3 4 5.

• Conceptual systems
• Management cybernetics
• Operational research
• Systems Engineering