become correlated with the state of after the measurement, and this correlation can take one of two values: or where the direction of the arrows in the subscripts corresponds to the outcome of the measurement that has made on . If we now consider the description of the measurement event by the other observer, , who describes the combined system, but does not interact with it, the following gives the description of the measurement event according to , from the linearity inherent in the quantum formalism: Thus, on the assumption (see hypothesis 2 below) that quantum mechanics is complete, the two observers and give different but equally correct accounts of the events . Note that the above scenario is directly linked to Wigner's Friend thought experiment, which serves as a prime example when understanding different interpretations of quantum theory. Central principles Observer-dependence of state According to , at , the system is in a determinate state, namely spin up. And, if quantum mechanics is complete, then so is this description. But, for , is not uniquely determinate, but is rather entangled with the state of note that his description of the situation at is not factorisable no matter what basis chosen. But, if quantum mechanics is complete, then the description that gives is also complete. Thus the standard mathematical formulation of quantum mechanics allows different observers to give different accounts of the same sequence of events. There are many ways to overcome this perceived difficulty. It could be described as an epistemic limitation observers with a full knowledge of the system, we might say, could give a complete and equivalent description of the state of affairs, but that obtaining this knowledge is impossible in practice. But whom? What makes 's description better than that of , or vice versa? Alternatively, we could claim that quantum mechanics is not a complete theory, and that by adding more structure we could arrive at a universal description (the troubled hidden variables approach). Yet another option is to give a preferred status to a particular observer or type of observer, and assign the epithet of correctness to their description alone. This has the disadvantage of being ad hoc, since there are no clearly defined or physically intuitive criteria by which this super-observer ("who can observe all possible sets of observations by all observers over the entire universe") ought to be chosen. RQM, however, takes