  A generalization of the action principle of classical mechanics, motivated by
the Closed Time Path (CTP) scheme of quantum field theory, is presented to deal
with initial condition problems and dissipative forces. The similarities of the
classical and the quantum cases are underlined. In particular, effective
interactions which describe classical dissipative forces represent the
system-environment entanglement. The relation between the traditional effective
theories and their CTP extension is briefly discussed and few qualitative
examples are mentioned.
