  Macroscopic quantum phenomena (MQP) is a relatively new research venue, with
exciting ongoing experiments and bright prospects, yet with surprisingly little
theoretical activity. What makes MQP intellectually stimulating is because it
is counterpoised against the traditional view that macroscopic means classical.
In this series of papers we report on a systematic investigation into some key
foundational issues of MQP, with the hope of constructing a viable theoretical
framework for this new endeavour. The three major themes discussed in these
three essays are the large N expansion, the correlation hierarchy and quantum
entanglement for systems of `large' sizes, with many components or degrees of
freedom. In this paper we use different theories in a variety of contexts to
examine the conditions or criteria whereby a macroscopic quantum system may
take on classical attributes, and, more interestingly, that it keeps some of
its quantum features. The theories we consider here are, the O(N) quantum
mechanical model, semiclassical stochastic gravity and gauge / string theories;
the contexts include that of a `quantum roll' in inflationary cosmology,
entropy generation in quantum Vlasov equation for plasmas, the leading order
and next-to-leading order large N behaviour, and hydrodynamic / thermodynamic
limits. By examining different representative systems where detailed
theoretical analysis has been carried out, we find that there is no a priori
good reason why quantum phenomena in macroscopic objects cannot exist.
