I would describe complex systems as multivariable nonlinear systems. And multivariable nonlinear systems are very hard to predict. They can behave one way for a long time, and then one variable can switch and they can behave another way—the weather, stock markets, all these things. There are consequences that can be first, second, or third-order. You can't just think with a linear model or just think about one variable because things can go way off the path. You need to be aware that if you make a change here, it could change something here, which could change something there, and it has to be the whole system.
