The Horizontal Symmetry for Neutrino Mixing

We argue that the best way to determine horizontal symmetry is from neutrino mixing, and proceed to show that the only finite group capable of yielding the tri-bimaximal mixing for all Yukawa couplings is $S_4$, or any group containing it. The method used is largely group theoretical, but it can be implemented by dynamical schemes in which the Higgs expectation values introduced to break $S_4$ spontaneously are uniquely determined up to an unknown scale for each.