  The notion of state reduction employed by the standard quantum theory of
measurement is difficult to accept for two reasons: It leaves open where and
when the reduction takes place and it does not give any objective conditions
under which the reduction occurs. Some recently published ideas on this problem
are developed an improved. The disturbance of measurement due to identical
particles in the environment is shown to make any POV measure non-measurable.
Truncated POV (TPOV) measures are introduced that can be measurable if object
systems satisfy the additional requirement of having separation status. The
separation status is generalised from domain of space to domain of phase space.
Starting from the previously introduced distinction between ancillas and
detectors, further study of experiments suggests that a thermodynamic mixing
within a detector and the consequent loss of separation status is the objective
condition for the occurrence of the state reduction. The conjecture is simple,
specific and testable. The theory is illustrated by a model of a real
measurement.
