  The measuring process is an external intervention in the dynamics of a
quantum system. It involves a unitary interaction of that system with a
measuring apparatus, a further interaction of both with an unknown environment
causing decoherence, and then the deletion of a subsystem. This description of
the measuring process is a substantial generalization of current models in
quantum measurement theory. In particular, no ancilla is needed. The final
result is represented by a completely positive map of the quantum state $\rho$
(possibly with a change of the dimensions of $\rho$). A continuous limit of the
above process leads to Lindblad's equation for the quantum dynamical semigroup.
