Hey guys, Hope all is well. I'm trying to get my head round some of the Quantum Mechanics of spin. I fully understand why the Pauli equation acts on a two component spinor wavefunction, where i'm a little confused is why the Dirac equation then acts on a 4 component spinor wavefunction. I get that it is in a sense four coupled equations but why is spin implied? I've done a fair bit of reading in the griffiths book but any extra insight would be very much appreciated.