  We synthesize and partly review recent developments relating the physics of
the half-filled Landau level in two dimensions to correlated surface states of
topological insulators in three dimensions. The latter are in turn related to
the physics of certain three dimensional quantum spin liquid states. The
resulting insights provide an interesting answer to the old question of how
particle-hole symmetry is realized in composite fermion liquids. Specifically
the metallic state at filling $\nu = \frac{1}{2}$ - described originally in
pioneering work by Halperin , Lee, and Read as a liquid of composite fermions -
was proposed recently by Son to be described by a particle-hole symmetric
effective field theory distinct from that in the prior literature. We show how
the relation to topological insulator surface states leads to a physical
understanding of the correctness of this proposal. We develop a simple picture
of the particle-hole symmetric composite fermion through a modification of
older pictures as electrically neutral "dipolar" particles. We revisit the
phenomenology of composite fermi liquids (with or without particle-hole
symmetry), and show that their heat/electrical transport dramatically violates
the conventional Wiedemann-Franz law but satisfies a modified one. We also
discuss the implications of these insights for finding physical realizations of
correlated topological insulator surfaces.
