There is a // plate capacitor with one plate charged (sigma) and the other charged (-sigma). Between the plates are 2 dielectrics each of thickness a. (This does not matter to my question). Why is the electric displacement equal to sigma? Every solution I find draws a gaussian surface around one plate and says DA=(sigma)A, so D=sigma. I can accept that, but it seems to me that that neglects the contribution of the second plate which should yeild a total D=2*sigma (by my incorrect thinking). Why do we only need the surface to be around one plate?