  Physical systems are characterized by their structure and dynamics. But the
physical laws only express relations, and their symmetries allow any possible
relational structure to be also possible in a different parametrization or
basis of the state space. If observers were reducible to their structure,
observer-like structures from different parametrizations would identify
differently the observables with physical properties. They would perceive the
same system as being in a different state. This leads to the question: is there
a unique correspondence between observables and physical properties, or this
correspondence is relative to the parametrization in which the observer-like
structure making the observation exists?
  I show that, if observer-like structures from all parametrizations were
observers, their memory of the external world would have no correspondence with
the facts, it would be no better than random guess. Since our experience shows
that this is not the case, there must be more to the observers than their
structure. This implies that the correspondence between observables and
physical properties is unique, and it becomes manifest through the observers.
This result is independent of the measurement problem, applying to both quantum
and classical physics. It has implications for structural realism, philosophy
of mind, the foundations of quantum and classical physics, and quantum-first
approaches.
