Schweser - Study Notes- LOS 76.f

On page 165 bottom, where Schweser starts to set out a proof, the third component of this is a short position in one share of underlying stock priced S sub zero = X Why is the current value of that position necessarily negative S sub zero? based on this, there is no scenario under which your short position could earn you any profit? why aren’t they taking account into this formula your basis in the short?