 (b) The second assumption of classical mechanics which is dropped in RQM is that there are variables that take absolute values, namely values independent from any other systems. Instead, RQM assumes that all (contingent) physical variables are relational. (The contingent variables are those with in classical theory are represented by phase space functions.) Any value these variables take is always (implicitly or explicitly) labelled by a second physical system. If the variable \(A\) of a system \(S\) takes a value in the interaction with a second systems \(S'\), the value it takes is only relative to \(S'\). The actualisation of an event is always relative to a system. The concrete meaning of this is the postulate above, according to which future ways the system \(S\) affects \(S'\) depend (probabilistically) on the values that variables of \(S\) have taken with respect to \(S'\), but future ways the system \(S\) affect a third system \(S''\) do not. The ensemble of all events relative to an arbitrary system \(S'\), together with the probabilistic predictions these entail, is called the “perspective” of the “observer” \(S'\).