The whole is equal to the sum of the parts.
The whole is greater than the sum of the parts.
These expressions represent two very different types of systems.
When someone buys a newspaper at the corner drugstore, it has no effect on my decision to buy a tube of toothpaste at the supermarket. The two events are independent. The whole is equal to the sum of the parts.
If lots of people start buying various products then it may influence others to buy products. This reinforcing behavior may create a boom in the economy. Conversely, if lots of people stop buying products then it may influence others to save, and thus create a recession. The actions by the individuals are dependent. The whole is greater than the sum of the parts.
Systems where the whole is equal to the sum of the parts are called linear systems. Each component is independent of the others.
Systems where the whole is greater than the sum of the parts are called non-linear systems. Each component may influence other components. A non-linear system is a vast web of incentives and constraints and connections. The slightest change in one part causes tremors everywhere else. “We can’t help but disturb the Universe”
– Extracted from Complexity by M. Mitchell Waldrop