In tbis lecture, Professor Susskind reviews the nature of probability densities and how they can represent states of systems when we only partial, or probabilistic information about the state of the system.

We then generalise density to composite systems and consider the relationship between entropy - specifically **entanglement entropy** - and the probability density.

Finally, we posulate how states evolve over time. We define the evolution operator and the Hamiltonian, and introduce **Shrodinger's equation**.