 None of the standard formulations are precise about when the collapse postulate is to be applied; there is some lee-way as to what is to count as an experiment, or (for versions that require reference to an observer) what is to count as an observer. Some, including von Neumann and Heisenberg, have taken it to be a matter of principle that there be some arbitrariness in where to apply the postulate. It is common wisdom that, in practice, this arbitrariness is innocuous. The rule of thumb that seems to be applied, in practice, in setting the split between the parts of the world treated quantum-mechanically and things treated as classical objects has been formulated by J. S. Bell as, “[w]hen in doubt enlarge the quantum system,” to the point at which including more in the quantum system makes negligible difference to practical predictions (Bell 1986, 362; Bell 2004, 189). If anything is to be counted as “standard” quantum mechanics, it is the operational core we have discussed, supplemented by a heuristic rule of application of this sort. Standard quantum mechanics works very well.  If, however, one seeks a theory that is capable of describing all systems, including macroscopic ones, and can yield an account of the process by which macroscopic events, including experimental outcomes, come about, this gives rise to the so-called “measurement problem”, which we will discuss after we have introduced the notion of entanglement (see   section 3).