We recall a refinement of the Lax–Milgram theorem from Trostorff–Waurick in 2014, which highlights the particular role of the coefficient in divergence form problems forming the crucial point leading to whether or not the respective PDEs admits a unique solution continuously depending on the data. This insight leads to the concept of the inner spectrum introduced in this chapter. The relationship to the spectrum of the Neumann–Poincaré operator is provided.