  Quasars are rapidly accreting supermassive black holes at the center of
massive galaxies. They display a broad range of properties across all
wavelengths, reflecting the diversity in the physical conditions of the regions
close to the central engine. These properties, however, are not random, but
form well-defined trends. The dominant trend is known as Eigenvector 1, where
many properties correlate with the strength of optical iron and [OIII]
emission. The main physical driver of Eigenvector 1 has long been suspected to
be the quasar luminosity normalized by the mass of the hole (the Eddington
ratio), an important quantity of the black hole accretion process. But a
definitive proof has been missing. Here we report an analysis of archival data
that reveals that Eddington ratio indeed drives Eigenvector 1. We also find
that orientation plays a significant role in determining the observed
kinematics of the gas, implying a flattened, disklike geometry for the
fast-moving clouds close to the hole. Our results show that most of the
diversity of quasar phenomenology can be unified with two simple quantities,
Eddington ratio and orientation.
