What speakers mean by it
In current English the noun covers two clusters that feel distinct until you look at the history. A reason is a cause, a motive, or an explanation offered for a choice. Reason, without an article, is also the faculty of thinking things through: judgment, inference, the power people contrast with impulse or faith. The verb tracks the same split. To reason is to work out an argument; to reason with someone is to try to move them by talk rather than force.
Those two jobs — motive and mind — were already packed into the word when English borrowed it. They did not have to be invented later as metaphors.
A French loan on an English island
The origin of the word reason is a borrowing, not a native survival from Old English. Around 1200 it shows up in Middle English as resoun or reson, taken from Anglo-Norman raisun and Old French raison. In French the noun already ranged widely: a course of action, a subject, speech, thought, opinion. That breadth is why English could treat the new word as both an account of why something happened and a name for the mind that gives such accounts.
The French form continues Latin ratio, usually through the accusative rationem, the case Romance languages commonly inherited. In Latin the core was reckoning and calculation, then understanding, motive, and cause. English later borrowed the same Latin noun again in learned dress: ratio, mainly for proportion, in the seventeenth century, and ration for a measured allowance. Those are doublets — one Latin source, different centuries and registers.
Independent dictionaries place the English noun in the thirteenth century, which matches the Middle English spellings. Once it settled, it crowded out native words that had done similar work. Old English racu could mean an account or explanation. Old English rǣden covered calculation, stipulation, and direction, and a related Middle English reden lingered in compounds. Dutch reden and West Frisian reden still show the Germanic line English mostly abandoned for this sense.
From the account-book to the mind
Behind Latin ratio stands the verb reor, infinitive reri: to reckon, to calculate, and by a short step to think or believe. The past participle ratus supplied the stem. The move from counting to considering is ordinary in this family. To give an account of something is already close to explaining it; to keep a reckoning is already close to judging.
Etymologists push the Latin verb further back to a Proto-Indo-European root reconstructed with the sense of counting and thinking. Other handbooks treat that root as a split from an older one meaning to fit things together; the American Heritage tradition points to the Indo-European root written ar-. The notations differ. They do not disagree about the Latin and French steps that actually produced the English noun.
From the Latin cluster English also has rational, ratify, rate, and ratiocination. On the Germanic side, from the broader root rather than from Latin, sit read and the older sense of riddle as counsel or interpretation. Gothic rathjo, glossed as account or explanation, belongs to that Germanic branch. The family keeps returning to the same picture: numbers, counsel, and explanation as kinds of fitting things into order.
In philosophical Latin, ratio often stood in for Greek logos when a translator needed a word for reckoning, account, and reasoned thought. The overlap is real and incomplete. Logos also means speech and word, a range the Latin noun never fully covered. English later borrowed logos on its own, so the two trails remain separate even when philosophy treats them as neighbours.
How the senses moved
By about 1200 the English noun already named the intellectual faculty that fits means to ends, and also a statement offered in argument or justification. Late in the thirteenth century it is recorded for sanity: the degree of intelligence thought to set humans apart from other animals. Around 1300 two everyday senses take hold that still dominate: grounds for an action, and a view that commends itself to sound judgment.
Early in the fourteenth century the word could also mean signification, the sense of a thing. That use is mostly gone from free prose, but it survives in the set phrase rhyme or reason, and in rhyme nor reason: neither sound-pattern nor intelligible meaning. Reason of state, from the early 1600s, is a colder offshoot, a political motive offered as if it were necessity.
The verb is later than the noun. About 1400, from Old French resoner or raisoner and Late Latin rationare, to discourse, English resounen meant to question or challenge someone. Only in the 1590s is the intransitive sense of thinking in a logical way firmly recorded. To reason with someone in the modern sense is later still, from the 1680s. Stand to reason, from 1620, grew out of an earlier stands with reason: the claim is said to hold to, or agree with, what is reasonable.
An obsolete mathematical use treated the noun as ratio or proportion, which is simply the Latin meaning leaking back in. The Enlightenment then sharpened reason into something like a shared light: intelligence treated as universal rather than as one person’s talent. Age of Reason is already the title of Thomas Paine’s book in 1794. Personifications of Reason as a woman belong to that cultural habit, not to the etymology itself.
The proverb still keeps an older sense
If you want one dating that still explains ordinary speech, take the early fourteenth-century sense of meaning. It is why rhyme or reason pairs sound with sense. In that phrase the word is not a motive; it is intelligibility. The everyday name for a cause grew out of the same Latin account-book, but the proverb is where the older layer still shows.