Systems thinking is more than a theory or a scientific trend. Systems thinking is an idea, an understanding that reality organizes itself into systems. All the tiny different parts of reality—regardless of whether you cut up the pie into atoms, physical forces, or currents of social change—are interconnected to one another. They all interact in chaotic, highly unpredictable ways.
Systems thinking was designed to try to explain, model, and predict how “stuff” that seems to be completely unrelated from other “stuff”—like externalities—interact to create highly complex behavior: creating a system. Strictly speaking, a system isn’t built out of parts; it’s built out of interactions. It’s possible to have a collection of parts, but as long as they’re not interacting with each other in some particular way—as long as they don’t form part of a structure (a dynamic structure in this case)—they aren’t a system.
Therefore, systems thinking allows us to model how different “chunks” of reality interact.