  Spin Hall effect (SHE), a mechanism by which materials convert a
\textit{charge} current into a \textit{spin} current, invokes interesting
physics and promises to empower transformative, energy-efficient memory
technology. However, fundamental questions remain about the essential factors
that determine SHE. Here we solve this open problem, presenting a comprehensive
theory of five \textit{foundational factors} that control the value of
intrinsic SHE in transition metal oxides. Arising from our key insight
regarding the inherently geometric nature of SHE, we demonstrate that two of
these factors are crystal field strength and structural distortions. Moreover,
we discover that a new class of materials (anti-perovskites) promises to
demonstrate \textit{giant} SHE, that is an order of magnitude larger than that
reported for any oxide. We derive three other factors that control SHE and
demonstrate the nuanced role of electron correlations. Our findings bring
deeper insight into the physics driving SHE, and could help enhance, as well
as, externally control SHE values.
