" A quantum state is a useful tool because it can do just that. This is our next principle: Given the quantum state of an isolated system at one time, there is a law that will predict the precise quantum state of that system at any other time. This law is called Rule 1. It is also sometimes called the Schrödinger equation. The principle that there is such a law is called unitarity. "

― Lee Smolin , Einstein's Unfinished Revolution: The Search for What Lies Beyond the Quantum