  We suggest to take the fluctuation-dissipation theorem of Callen and Welton
as a basis to study quantum dissipative phenomena (such as macroscopic quantum
tunneling) in a manner analogous to the Nambu-Goldstone theorem for spontaneous
symmetry breakdown. It is shown that the essential physical contents of the
Caldeira-Leggett model such as the suppression of quantum coherence by Ohmic
dissipation are derived from general principles only, namely, the
fluctuation-dissipation theorem and unitarity and causality (i.e., dispersion
relations), without referring to an explicit form of the Lagrangian. An
interesting connection between quantum tunneling with Ohmic dissipation and the
Anderson's orthogonality theorem is also noted.
