Lenoard Susskind's video courses on Clasical Mechanics and Quantum Mechanics, often mention convervation of information. Susskind likes to call it "the minus first law." In classical physics, it is Liouville's Theorum which tells us that the number of states is conserved in time evolutions. In quantum mechanics, time evolutions are unitary. And in time evolutions, the number of possible states is conserved. Susskind said that unitarity in quantum mechanics is analogous to Liouville's Theorum in classical mechanics. I'm having diffculty understanding that analogy. The Wikipedia article Unitarity (physics) didn't help, although it does say that unitarity means that the sum of the probabilities of all possibile outcomes equals 1. Thus if the evolution was not unitary, some possibilities would dissapear or new possibilities would be created. Is that the proper thought thread?