  We review the issues associated with the construction of neutrino flavor
states. We then provide a consistent proof that the flavor states are
approximately well-defined only if neutrinos are ultra-relativistic or the mass
differences are negligible compared to energy. However, we show that weak
interactions can be consistently described by only neutrino mass eigenstates.
Meanwhile, the second quantization of neutrino flavor fields generally has no
physical relevance as their masses are indefinite. Therefore, the flavor states
are not physical quantum states and they should simply be interpreted as
definitions to denote specific linear combinations of mass eigenstates involved
in weak interactions. We also briefly discuss the implication of this work for
the mixing between active and heavy sterile neutrinos.
