The behavior of simple systems and complex systems can be predicted. In-between systems: not so much.
The behavior of a lone ball, set in motion by a known force on a billiard table, can be measured or predicted rather accurately. The behaviors of two balls will be more difficult to measure or predict, but the task is somewhat manageable. As the number of objects in the system is further increased—to five, ten, a hundred balls—tracking and/or predicting the behaviors of the individual balls becomes increasingly difficult and eventually impossible.
But at a later point, predictability reenters the model, albeit in a different form. It will remain difficult or impossible to predict the individual behaviors of a million balls on an enormous billiard table, but we will be able to predict many average behaviors, such as how often one ball is likely to collide with other balls, how many balls will strike a side rail in a second, or the average distance a ball will move before striking another.
With regards to Jane Jacobs,
The Death and Life of Great American Cities
