reason: [13] Reason, together with rational, represent in English the ‘thinking’ aspects of the Latin verb rērī (it also meant ‘calculate’, and in that guise has given English rate, ration, etc). From it was derived the noun ratiō ‘thinking, calculation’ (source of English ratio and the rest). This spawned a Vulgar Latin variant *ratiōne, which passed into Old French as reisun – whence English reason. => rate, ratio, ration
