Why a thing is so, and the faculty that works such things out. Both the ground and the tool for finding it.
Old French raison, from Latin ratio (reckoning, calculation, reason), from reri (to reckon, to think); reason names both the ground for a thing and the faculty that works such grounds out, sharing one root with ratio and rate.
Why a thing is so, and the faculty that works such things out. The root is a reckoning in both senses — reason is not a mystery invoked, it is a calculation carried through.