  It is proven, without using the microscopic reversibility argument of
Onsager, that lossy reciprocal systems have a hidden time-reversal symmetry.
The key idea is that the dissipation channels of lossy dielectrics can be
mimicked by a distributed network of lossless transmission lines. It is
highlighted that the reciprocity of lossy dielectrics is fundamentally rooted
on the hidden time-reversal invariance and on linearity of the materials.
Furthermore, it is demonstrated that the upper-half plane response of
dissipative materials can be approximated as much as desired by the response of
some lossless material.
