Origins
The word 'reason' traces a path from the counting table to the philosopher's chair. It entered English in the thirteenth century from Old French 'raison' (thought, reason, cause, speech), from Latin 'ratiō' (a reckoning, a calculation, a method, an account, reason, understanding), from 'ratus,' the past participle of 'rērī' (to reckon, to think, to calculate), from PIE *h₂reh₁- (to reason, to count). The original sense was concrete and numerical — a ratiō was a sum, a tally, a calculated result — and the expansion to mean 'the faculty of orderly thought' is one of the great metaphorical leaps in the history of Western vocabulary.
Latin 'ratiō' was already extraordinarily polysemous in classical usage. Cicero used it to mean 'calculation,' 'method,' 'system,' 'theory,' 'reason,' 'argument,' 'ground,' and 'relation' — depending on context. This semantic breadth transferred to Old French 'raison,' which could mean 'reason,' 'speech,' 'cause,' 'justice,' and 'right.' Modern French preserves the duality: 'avoir raison' means 'to be right,' and 'la raison' means 'reason.' Spanish 'razón,' Italian 'ragione,' and Portuguese 'razão' carry similar ranges.
The English family from 'ratiō' includes four words that have diverged completely in everyday use but are etymologically identical: 'reason' (the faculty of thought), 'ratio' (a proportional relationship between quantities), 'ration' (a calculated allotment, especially of food), and 'rate' (a calculated measure or charge). Each preserves a different facet of the original 'reckoning' sense: reason is reckoning in the mind, a ratio is a reckoning between numbers, a ration is a reckoned share, and a rate is a reckoned price or speed.
Latin Roots
The philosophical history of 'reason' in English reflects centuries of debate about the nature of human thought. Medieval Scholastic philosophers used Latin 'ratiō' to distinguish discursive reasoning (step-by-step logical thought) from 'intellectus' (direct intuitive understanding). Aquinas argued that human beings reason (move from premise to conclusion in steps) while angels understand (grasp truth immediately without steps). Kant's distinction between 'Verstand' (understanding, the faculty that organizes experience) and 'Vernunft' (reason, the faculty that seeks ultimate principles) continues this tradition in German — and English 'reason' has been asked to cover both concepts.
The phrase 'the Age of Reason' (applied to the seventeenth and eighteenth centuries) crystallizes the word's elevation from 'counting' to 'the defining faculty of humanity.' Descartes, Spinoza, Leibniz, and the French Enlightenment philosophes placed ratiō — now fully abstract — at the center of human identity and social organization. To be rational was the highest human achievement; to be irrational was to fall below the human standard.
The double meaning of 'reason' — both 'the faculty of thought' and 'a cause or explanation' ('the reason for the delay') — is already present in Latin 'ratiō,' which could mean both 'reasoning' and 'ground, basis, motive.' The two senses are connected: a reason (cause) is what reasoning (the faculty) discovers. To give a reason is to offer the result of a reckoning.