Professor Susskind builds on the discussion of quantum harmonic oscillators from the last lecture to derive the higher order energy states and wave functions. He then moves on to discuss spin states of particles, and introduces the Pauli matrices, which account for the interaction of a particle's spin with an external magnetic field. By examining the energy levels of electrons in an atom, Pauli and others realized that only two electrons can be in any given state. This led both to the the exclusion principle, as well as the need for another state variable - spin - which allows two electrons in each energy level. Topics: Spin Pauli matrices Pauli exclusion principle