ABSTRACT
This chapter suggests that the two most important principles of condensed matter physics for students purposes are, first, broken symmetry, which tells their that what the order parameter is and what symmetry it breaks are the most vital questions. Second is the continuity principle, which tells students to search for the right simple problem when confronted with a complicated one. The idea is to imagine oneself starting from a completely noninteracting Fermi gas and gradually turning on the interactions between the particles. Landau's argument is that there will be a one-to-one correspondence between states before and after the adiabatic turning-on process, so that he can label the new states with the quantum numbers of the old and they will be in roughly the same order. In the case of Fermi-liquid theory, this program has been formally carried out: one can produce diagrammatic expressions for all of the renormalizations, of the mass and of the various vertices, contained in the theory.
