Originally Posted by **Pereos** What I fail to understand is how an equation can describe random and unpredictable behaviour. |

There is the law of large numbers, which states that when the number of trials for a given probabilistic situation is high, the result tends towards the expectation value. Another interpretation of this is that for random phenomena, the error associated with the actual state of a system versus its predicted one decreases as the system increases in size.

In physics systems this number of trials is often very high or involves many, many particles. The result is a bulk motion that is very predictable and there are equations describe the bulk motion. A good example of this is Brownian motion or diffusion. Statistics plays a big role.