ABSTRACT. Perfect knowledge of the many-body system is contained in the wavefunction but, as Schrodinger has already emphasized, best possible knowledge of a whole does not include the best possible knowledge of its parts. Separability from the point of view of quantum mechanics is discussed. General -entangled- systems are analyzed in term of knowledge. If the state vector is defined for a whole system its parts are described only by mixed density operators. Correlations violating Bell's inequality are necessary to avoid superluminal signaling and result from the lack of independent reality of subsystems. Model calculations on two separated atoms and on non-interacting gas show that the perfect knowledge of the whole system or the total wavefunction is not sufficient to calculate local properties without actually solving the local problem.

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