A complex system is a collection of diverse, interconnected, and interdependent agents that interact with each other and adapt to their environment.
In a complex system, the whole is qualitatively different from the sum of its parts. This is called emergence—global patterns and behaviors arise that cannot be predicted by looking at any single agent in isolation. Furthermore, these systems are non-linear: inputs are not proportional to outputs. A minor shift can trigger a massive phase transition (tipping points), while a major intervention might produce no effect at all.
This behavior makes complex systems fundamentally different from complicated ones. A Boeing 747 is complicated—it has millions of parts, but they interact in fixed, predictable ways. If you turn it off and on again, it behaves exactly the same. In contrast, a traffic jam, an ecosystem, or a human team is complex. Because the agents within them learn and adapt, the system’s future state cannot be calculated mathematically.
This is why we cannot control a complex system; we can only influence and shape the environment to shift the probabilities of its adaptation.