  In this work we adopt the point of view that the equations of motion
satisfied by a field are just a consequence of the representation space which
the field belongs to, and the discrete symmetries we impose on it. We
illustrate this view point by rederiving Dirac and Proca equations as
projectors over the subspaces with well defined parity of (1/2,0)+(0,1/2) and
(1/2,0)X(0,1/2) representations respectively. We formulate the equation of
motion corresponding to the identification of elementary systems with states in
the invariant subspaces of the squared Pauli-Lubanski operator and couple
minimally to electromagnetism the corresponding equation for the (s,0)+(0,s)
representation space using the gauge principle. We obtain g=1/s for particles
with arbitrary spin s as conjectured by Belinfante long ago.
