Just want to clarify my understanding of these things. Fermions obey the Pauli Exclusion Principal, meaning meaning only one particle can occupy a single state. This means the wave function is anti-symmetric under particle exchange. That's the part that isn't making much sense. Is it because if we have two particles designated by quantum numbers n, l, m1 and n, l, m2 and we swap the particles the only way for the states to be different is if m1 = -m2? Also, when we speak of the state of a system... Consider the basic problem of a spin 1/2 particle in a constant magnetic field (0, 0, B). H = γSz , γ = geB/2mc, or something like that.. We write H|E> = E|E> H2|E> = E2|E> From this we determine the eigenvalues E1 and E2, and the state at some time t is given by |ψ> = C+eaE1t|E1> + C-eaE2t|E2> What does this actually mean? Is it the energy at some later time t? The spin?