Etymology

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.

      • 1996, Daniel Clement Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life, page 198,

        There is a reason why so many should be symmetrical: The selective advantage in a symmetrical complex is enjoyed by all the subunits

   2. A motive for an action or a determination.

      The reason I robbed the bank was that I needed the money.

      • 1806, [Anonymous], Select Notes to Book XXI, in, Alexander Pope, translator, The Odyssey of Homer, volume 6, London, F.J. du Roveray, page 37,

        This is the reason why he proposes to offer a libation, to atone for the abuse of the day by their diversions.

      • 1881, Henry James, The Portrait of a Lady, chapter 10,

        Ralph Touchett, for reasons best known to himself, had seen fit to say that Gilbert Osmond was not a good fellow.

   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.
      • 1966, Graham Greene, The Comedians, Penguin Classics edition, ISBN 0140184945, page 14,

        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.

   4. proof, more or less decisive, for an opinion or a conclusion
2. The cognitive faculties, collectively, of conception, judgment, reasoning, and intuition; the ability to think.

   Mankind should develop reason above all other virtues.

   • 1970, Hannah Arendt, On Violence, ISBN 0156695006, page 62,

     And the specific distinction between man and beast is now, strictly speaking, no longer reason (the lumen naturale of the human animal) but science.

3. Something reasonable, in accordance with thought; justice.

   I was promised, on a time, To have reason for my rhyme. —Edmund Spenser

4. due exercise of the reasoning faculty
5. ratio; proportion.

Verb

1. To exercise the rational faculty; to deduce inferences from premises; to perform the process of deduction or of induction; to ratiocinate; to reach conclusions by a systematic comparison of facts.
2. Hence: To carry on a process of deduction or of induction, in order to convince or to confute; to formulate and set forth propositions and the inferences from them; to argue.
3. To converse; to compare opinions.
4. 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.

5. To support with reasons, as a request.
6. To persuade by reasoning or argument.

   to reason one into a belief; to reason one out of his plan

7. To overcome or conquer by adducing reasons; — with down.

   to reason down a passion

8. To find by logical process; to explain or justify by reason or argument; — usually with out.

   to reason out the causes of the librations of the moon