  In this work I show by a first principles calculation that quantum states
describing massive relativistic free spinning particles obey kinematical
conditions whose origin can be traced to parity as a good quantum number. These
conditions are at the root of the equations of motion and of the constraints
satisfied by the corresponding fields. In the massless limit, well defined
parity is lost but a symmetry emerges related to arbitrary changes in the
unphysical parity components. It is shown that this emergent symmetry is the
celebrated gauge symmetry.
