reason

Before reason became the polite name for clear thinking, it was basically bookkeeping for the mind. Latin ratus meant something like "reckoned" or "settled," which is why reason, ratio, and rational are family members: they all began with counting, then graduated to judgment. By the time Anglo-French resoun reached English around 1200, it could mean thought, speech, explanation, even the subject of a discussion — a pretty dramatic promotion for a word that started in accounting. The same root gave us rationality on one side and, through a different path, the very modern-sounding ratio on the other; one word became philosophy, the other stayed in math class. And in 1594, Hooker was already warning that people who demand a reason for everything can end up destroying reason itself — a neat reminder that the mind can be both the lantern and the moth.