Quasars in the 4D Eigenvector 1 Context: A stroll down memory lane
  Recently some pessimism has been expressed about our lack of progress in
understanding quasars over the 50+ year since their discovery. It is worthwhile
to look back at some of the progress that has been made - but still lies under
the radar - perhaps because few people are working on optical/UV spectroscopy
in this field. Great advances in understanding quasar phenomenology have
emerged using eigenvector techniques. The 4D eigenvector 1 context provides a
surrogate H-R Diagram for quasars with a source main sequence driven by
Eddington ratio convolved with line-of-sight orientation. Appreciating the
striking differences between quasars at opposite ends of the main sequence
(so-called population A and B sources) opens the door towards a unified model
of quasar physics, geometry and kinematics. We present a review of some of the
progress that has been made over the past 15 years, and point out unsolved
issues.
