SUBJECTIVE DECOHERENCE IN QUANTUM MEASUREMENTS

General results about restrictions on measurements from inside are app lied to quantum mechanics. They imply subjective decoherence: For an a pparatus it is not possible to determine whether the joint system cons isting of itself and the observed system is in a statistical state wit h or without interference terms; it is possible that the apparatus sys tematically mistakes the real pure state of the joint system for the d ecohered state. We discuss the relevance of subjective decoherence for quantum measurements and for the problem of Wigner's friend.