The Kondo problem is studied using the unitary Lie algebra of spin-singlet fermion bilinears. In the limit when the number of values of the spin $N$ goes to infinity the theory approaches a classical limit, which still requires a renormalization. We determine the ground state of this renormalized theory. Then we construct a quantum theory around this classical limit, which amounts to recovering the case of finite $N$. ; Comment: 3 figures