What is the origin of the word rational?

Answer 1

I'm guessing that it relates to ration, which is a measurement, thus `measured thinking`. But I'm just guessing because I'm afraid to use my Oxford Concise Dictionary anymore because it's on the verge of totally falling apart and I can't afford a bookbinder. I bet it's a Greek word, anyway. The World Book Dictionary (Thorndike Barnhart) gives the source of ratio, rational and reason as the Latin: ratio rationis: reckoning. Apparently, the starting point for the development of the word is in mathematics (Greek, perhaps) where a rational number is one that can be expressed as a ratio. That's already a strange step from "reckoning" but that seems to be the case. From there, it seems to leap to describing a quality that fits or makes sense or is measured or orderly.