Systems Theory is the interdisciplinary study of systems — cohesive arrangements of interrelated, interdependent parts bounded in space and time — and of the general principles governing their behavior, regardless of substrate.
A system is described by its structure, environment, and function, and is more than the sum of its parts when it exhibits 📝synergy or emergent behavior. Changes to one element propagate through the whole in patterns that can often be characterized; self-learning and self-adapting systems either succeed or fail according to how well they remain coupled with their environment.
Niklas Luhmann's social-systems variant treats society itself as a system of communications, with the interior of each subsystem reducing complexity by selecting from the noise outside. Function systems — economy, law, science, media — are operationally closed: they rely on environmental resources without absorbing them into their own operations, and they read other systems only through their own codes. The field is the broader academic context within which 📝Systems Thinking operates as the applied frame.