1630s, in theological writing, "reason, rationale," from Latin ratio "a reckoning, account, a numbering, calculation," hence also "a business affair; course, conduct, procedure," also in a transferred sense, of mental action, "reason, reasoning, judgment, understanding, that faculty of the mind which forms the basis of computation and calculation." This is from rat-, past-participle stem of reri "to reckon, calculate," also "to think, believe" (from PIE root *re- "to think, reason, count").

Latin ratio often was used to represent or translate Greek logos ("computation, account, esteem, reason") in works of philosophy, though the range of senses in the two do not overlap (ratio lacks the key "speech, word, statement" meaning in the Greek word; see Logos).

The mathematical sense of "relation between two similar magnitudes in respect to quantity," measured by the number of times one contains the other, is attested in English from 1650s (it also was a sense in Greek logos). The general or extended sense of "corresponding relationship between things not precisely measurable" is by 1808.

1550, "the mental process of reasoning," later, "the relation of one number to another" (1660s), then "fixed allowance of food or other means of subsistence for a fixed period of time" (1702, often rations, from French ration in this sense).

All are from Latin rationem (nominative ratio) "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," in Medieval Latin "a computed share or allowance of food." This is from rat-, past participle stem of reri "to reckon, calculate," also "think" (from PIE root *re- "to reason, count").

The first sense listed for the English word is obsolete; the second has gone with ratio. The military pronunciation (rhymes with fashion) took over in English from the preferred civilian pronunciation (rhymes with nation) during World War I. That war also gave the word a specific sense of "officially limited allowance for civilians in times of war or dearth" (by 1917).

c. 1200, resoun, "the intellectual faculty that adopts actions to ends," also "statement in an argument, statement of explanation or justification," from Anglo-French resoun, Old French raison "course; matter; subject; language, speech; thought, opinion," from Latin rationem (nominative ratio) "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think" (from PIE root *re- "to reason, count").

The meaning "sanity; degree of intelligence that distinguishes men from brutes" is recorded from late 13c.; that of "that which recommends itself to enlightened intelligence, a reasonable view of a matter" is from c. 1300.

The sense of "grounds for action, motive, cause of an event" is from c. 1300. The Middle English sense of "meaning, signification" (early 14c.) is preserved in the phrase rhyme or reason. For stands to reason see stand (v.). A reason of state (1610s) is a purely political grounds for action.

The Enlightenment gave reason its focused sense of "intelligence considered as having universal validity ... so that it is not something that belongs to any person, but is something partaken of, a sort of light in which every mind must perceive" [Century Dictionary]. Reason itself has long been personified, typically as a woman. Age of Reason "the European Enlightenment" is by 1794 as the title of Tom Paine's book.

"They that seek a reason of all things, do utterly overthrow reason." [Hooker, translating Theophrastus ('Metaphysics'), in "Ecclesiastical Polity," 1594]

Reason is never a root, neither of act nor desire.

[Robinson Jeffers, "Meditation on Saviors"]

also a-rational, "not purporting to be governed by laws of reason," 1935; see a- (2) + rational.

late 15c., "not endowed with reason" (of beasts, etc.), from Latin irrationalis/inrationalis "without reason, not rational," from assimilated form of in- "not, opposite of" (see in- (1)) + rationalis "of or belonging to reason, reasonable" (see rational (adj.)).

Meaning "illogical, absurd" is attested from 1640s. Related: Irrationally. The mathematical sense "inexpressible in ordinary numbers" is from late 14c. in English, from use of the Latin word as a translation of Greek alogon in Euclid.

1650s, "exposition of principles," from Late Latin rationale, noun use of neuter of Latin rationalis "of reason" (see rational). Hence, "fundamental reason, the rational basis or motive of anything" (1680s).

1620s, "one who follows reason and not authority in thought or speculation," especially "physician whose treatment is based on reasoning," from rational + -ist. In theology/philosophy, "one who applies rational criticism to the claims of supernatural authority or revelation," 1640s. This sense shades into that of "one who believes that human reason, properly employed, renders religion superfluous." Related: Rationalistic; rationalism (1800 in medicine; 1827 in theology, "adherence to the supremacy of reason in matters of belief or conduct;" by 1876 in general use).

1620s, "quality of having reason;" 1650s, "fact of being agreeable to reason," from French rationalité and directly from Late Latin rationalitas "reasonableness, rationality" (also source of Spanish racionalidad, Italian razionalita), from Latin rationalis "of or belonging to reason, reasonable" (see rational). Middle English had racionabilite "the faculty of reason" (early 15c.), from Latin rationabilitas.

1767, "explain in a rational way, make conformable to reason," from rational + -ize. The psychological sense of "to give an explanation that conceals true motives" dates from 1922, on the notion of "cause to appear reasonable or socially acceptable." In 19c. Newman and Browning also used it with an intransitive sense of "think for oneself, employ one's reason as the supreme test." Related: Rationalized; rationalizer; rationalizing.

also superrational, "that is above or beyond the scope of reason," 1680s, from super- + rational (adj.).