reason

Etymology

From Middle English resoun, reson, from Anglo-Norman raisun (Old French raison), from Latin ratiō, from ratus, past participle of reor (“reckon”), ultimately from Proto-Indo-European *h₂reh₁- (“to think”), reanalysed root of *h₂er- (“to put together”). Doublet of ration and ratio.

noun

1. A cause:
   1. That which causes something: an efficient cause, a proximate cause.
      The reason this tree fell is that it had rotted.
      There is a reason why so many should be symmetrical: The selective advantage in a symmetrical complex is enjoyed by all the subunits[…] 1996, Daniel Clement Dennett, Darwin’s Dangerous Idea: Evolution and the Meanings of Life, page 198
   2. A motive for an action or a determination.
      The reason I robbed the bank was that I needed the money.
      If you don't give me a reason to go with you, I won't.
      This is the reason why he proposes to offer a libation, to atone for the abuse of the day by their diversions. 1806, Anonymous, Select Notes to Book XXI, in, Alexander Pope, translator, The Odyssey of Homer, volume 6 (London, F.J. du Roveray), page 37
   3. An excuse: a thought or a consideration offered in support of a determination or an opinion; that which is offered or accepted as an explanation.
      I have forgotten the reason he gave for not travelling by air. I felt sure that it was not the correct reason, and that he suffered from a heart trouble which he kept to himself. 1966, Graham Greene, The Comedians, Penguin Classics, page 14
   4. (logic) A premise placed after its conclusion.
2. (uncountable) Rational thinking (or the capacity for it); the cognitive faculties, collectively, of conception, judgment, deduction and intuition.
   Mankind should develop reason above all other virtues.
   The tremendous tragedy in which he had been involved - it was evident he was a fugitive from Weybridge - had driven him to the very verge of his reason. 1898, H.G. Wells, The War of the Worlds, London: William Heinemann, page 113
   And the specific distinction between man and beast is now, strictly speaking, no longer reason (the lumen naturale of the human animal) but science[…] 1970, Hannah Arendt, On Violence, page 62
   The [Isaac] Newton that emerges from the [unpublished] manuscripts is far from the popular image of a rational practitioner of cold and pure reason. The architect of modern science was himself not very modern. He was obsessed with alchemy. 2014-06-21, “Magician’s brain”, in The Economist, volume 411, number 8892
3. (obsolete) Something reasonable, in accordance with thought; justice.
   16th century Edmund Spenser, Lines on his Promised Pension I was promised, on a time, To have reason for my rhyme.
4. (mathematics, obsolete) Ratio; proportion.
   {{RQ:Barrow Mathematical Learning|chapter=Lecture XVII. Of the Names and Diversities of the Twofold Kind of Reason or Proportion, viz. Arithmetical and Geometrical|pages=323–324|pageref=324|passage=[I]f two Quantities repreſented by the Numbers 20 and 4 be compared, by dividing the Antecedent 20 by the Conſequent 4, the Quotient is 5; but inverting the Terms, by dividing 4 by 20 the Quotient is tfrac 4 20= tfrac 1 5. By which Quotients are declared the Geometrical Reaſons of the propoſed Quantities, becauſe if the Quotient found be multiplied by the Conſequent, the Product is equal to the Antecedent; for in the former Compariſon 5×4=20, in the latter tfrac 1 5×20=4; as Things again are referred to Equality.}}

verb

1. (intransitive) To deduce or come to a conclusion by being rational
   "I had," said he, "come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data. […]" 1892, Arthur Conan Doyle, The Adventure of the Speckled Band
2. (intransitive) To perform a process of deduction or of induction, in order to convince or to confute; to argue.
3. (intransitive, obsolete) To converse; to compare opinions.
4. (transitive, intransitive) To arrange and present the reasons for or against; to examine or discuss by arguments; to debate or discuss.
   I reasoned the matter with my friend.
   The talk was mainly between Aleck and Murdie, the others crowding eagerly about and putting in a word as they could. Murdie was reasoning good-humoredly, Aleck replying fiercely. 1901, Ralph Connor, chapter 9, in The Man from Glengarry
5. (transitive, rare) To support with reasons, as a request.
6. (transitive) To persuade by reasoning or argument.
   to reason one into a belief; to reason one out of his plan
7. (transitive, with down) To overcome or conquer by adducing reasons.
   to reason down a passion
8. (transitive, usually with out) To find by logical process; to explain or justify by reason or argument.
   to reason out the causes of the librations of the moon