We discuss the origin of dissipation in a one-dimension model describing the interaction of a microsystem (an oscillator) with a bath (a quantized field). The Hamiltonian is a gauge-type coupling of the oscillator with the field and it is bounded below. Classical and quantum pictures are considered. Our formulation of the problem: what stable states are described by the total Hamiltonian if the excited states of the oscillator are unstable? How can these unstable states arise in a conservative system? The vacua of the free and the interacting systems are found in dipole approximation. The theory determines a formfactor which optimizes the contributions of the total Hamiltonian in dipole approximation. These two vacua generate equivalent representations of canonical commutation relations. As a result of the oscillatorfield interaction the stable states of this system consist of the vacuum (oscillator ground state) and quanta of the quantized field (bath). It means that the oscillator as a stable state can exist only in the ground state. Any excited oscillator states can be seen as resonances in the field-field scattering. 1994 Academic Press, Inc.