  We present a prescription for consistently constructing non-Fock coherent
flavour neutrino states within the framework of the seesaw mechanism, and
establish that the physical vacuum of massive neutrinos is a condensate of
Standard Model massless neutrino states. The coherent states, involving a
finite number of massive states, are derived by constructing their creation
operator. Such a construction is the key requirement so that the oscillations
of particles indeed occur. We comment on the inherent non-unitarity of the
oscillation probability induced by the requirement of coherence.
