  The relational approach to quantum states asserts that the physical
description of quantum systems is always relative to something or someone. In
relational quantum mechanics (RQM) it is relative to other quantum systems, in
the (neo-)Copenhagen interpretation of quantum theory to measurement contexts,
and in QBism to the beliefs of the agents. In contrast to the other two
interpretations, in RQM any interaction between two quantum systems counts as a
"measurement", and the terms "observer" and "observed system" apply to
arbitrary systems. We show, in the form of a no-go theorem, that in RQM the
physical description of a system relative to an observer cannot represent
knowledge about the observer in the conventional sense of this term. The
problem lies in the ambiguity in the choice of the basis with respect to which
the relative states are to be defined in RQM. In interpretations of quantum
theory where observations play a fundamental role, the problem does not arise
because the experimental context defines a preferred basis.
