1. Complex systems are fundamentally different from simple systems that have been the focus of science since Newton's time. The interaction between simple systems is weak, such as enclosed gas or distant galaxies, so we can study their behavior by simple statistical averaging method.

2. The complexity is not necessarily proportional to the scale of the system. Complex systems must have a certain scale. Individuals in complex systems generally have certain intelligence, such as cells in organizations, investors in the stock market, and drivers in urban transportation systems. These individuals can make intelligent judgments or decisions according to their own environment through their own rules.

3. Descriptive definition of complex system: A complex system is a system with a moderate number of intelligent and adaptive agents who act according to local information.