Etymology
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reason (v.)

c. 1400, resounen, "to question (someone)," also "to challenge," from Old French resoner, raisoner "speak, discuss; argue; address; speak to," from Late Latin rationare "to discourse," from Latin ratio "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think" (from PIE root *re- "to reason, count").

The intransitive sense of "to think in a logical manner, exercise the faculty of reason" is from 1590s; transitive sense of "employ reasoning (with someone)" is from 1680s. Related: Reasoned; reasoning.

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reason (n.)

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").

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. Phrase it stands to (or with) reason is from 1520s. 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 first recorded 1794 as the title of Tom Paine's book. 

Reason is never a root, neither of act nor desire.
[Robinson Jeffers, "Meditation on Saviors"]
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relation (n.)

c. 1300, relacioun, "relationship, connection, correspondence;" late 14c. as "act of telling or relating in words," from Anglo-French relacioun, Old French relacion "report, connection" (14c.) and directly from Latin relationem (nominative relatio) "a bringing back, restoring; a report, proposition," from relatus (see relate).

The meaning "person related by blood or marriage" is attested from c. 1500. The phrase no relation "not in the same family," used in differentiating persons with the same surname, is attested by 1930.

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causation (n.)

1640s, "act of causing or producing," noun of action from cause (v.), or else from Medieval Latin causationem (nominative causatio) "action of causing" (in classical Latin "excuse, pretext"), from Latin causa "a cause, reason" (see cause (n.)). Meaning "relation of cause to effect" is from 1739.

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theocrat (n.)
1827, "a ruler in the name of God," from Greek theos "god" (from PIE root *dhes-, forming words for religious concepts) + -crat, from aristocrat, etc. From 1843 as "one who favors a system of theocracy." Theocratist was the name of a publication begun in 1828 "to maintain the essential relation which subsists between religion and politics," and might be used in the sense "one who emphasizes divine authority over reason and individual freedom and who explains social order as a revelation from God."
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correlation (n.)

1560s, "mutual relation, interdependence, interconnection," from French corrélation, from cor- "together" (see com-) + relation (see relation). Meaning "action of bringing into orderly connection" is by 1879.

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ratio (n.)

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.

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logocentric (adj.)
"centered on reason," 1931, from logo- "reason" + -centric.
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relational (adj.)

1660s, "of or pertaining to human relations," from relation + -al (1). From 1840 as "indicating or specifying some relation" in general. Related: Relationality.

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