Ratio
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
Rate; proportion; degree. Reason or understanding. Also a cause or giving judgment therein.
— Ratio decidendi. The ground of decision. The point in a case which determines the judgment.
— Ratio legis. The reason or occasion of a law ; the occasion of making a law. BL Law Tracts, 3.