means The mental process of drawing conclusions, forming judgments, or working out a problem by connecting one thought logically to the next.

from From the verb 'reason,' which came into English through Old French 'raisoner' from Latin 'ratio'—meaning calculation, account, or judgment—itself tied to 'reri,' to reckon or think. The same Latin root branches out into 'ratio,' 'rational,' and 'rate,' all carrying that ancient flavor of measuring and accounting. So to reason was once, quite literally, to do the math of thought.