The four rest-frame spinors u ( s ) ( 0 → → ) , v ( s ) ( 0 → → ) indicate that there are four distinct, real, linearly independent solutions to the Dirac equation. That they are indeed solutions can be made clear by observing that, when written in momentum space, the Dirac equation has the form ( p / − − m ) u ( s ) ( p → → ) = 0 and ( p / + m ) v ( s ) ( p → → ) = 0