 But the standard formulation of quantum mechanics does not say what it takes for an interaction to count as a measurement. Without specifying this, the theory is at best incomplete since it does not indicate when each dynamical law obtains. Moreover, if one supposes that observers and their measuring devices are constructed from simpler systems that each obey the deterministic dynamics, as Everett did, then the composite systems, the observers and their measuring devices, must evolve in a continuous deterministic way, and nothing like the random, discontinuous evolution described by rule 4b can ever occur. That is, if observers and their measuring devices are understood as being constructed of simpler systems each behaving as quantum mechanics requires, each obeying rule 4a, then the standard formulation of quantum mechanics is logically inconsistent since it says that the two systems together must obey rule 4b. This is the quantum measurement problem in the context of the standard collapse formulation of quantum mechanics. See the section on the measurement problem in the entry on  philosophical issues in quantum theory.