The Collaborative International Dictionary of English v.0.48:

Reason \Rea"son\ (r[=e]"z'n), n. [OE. resoun, F. raison, fr. L.
   ratio (akin to Goth. ra[thorn]j[=o] number, account,
   gara[thorn]jan to count, G. rede speech, reden to speak), fr.
   reri, ratus, to reckon, believe, think. Cf. Arraign,
   Rate, Ratio, Ration.]
   1. A thought or a consideration offered in support of a
      determination or an opinion; a just ground for a
      conclusion or an action; that which is offered or accepted
      as an explanation; the efficient cause of an occurrence or
      a phenomenon; a motive for an action or a determination;
      proof, more or less decisive, for an opinion or a
      conclusion; principle; efficient cause; final cause;
      ground of argument.
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            I'll give him reasons for it.         --Shak.
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            The reason of the motion of the balance in a wheel
            watch is by the motion of the next wheel. --Sir M.
                                                  Hale.
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            This reason did the ancient fathers render, why the
            church was called "catholic."         --Bp. Pearson.
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            Virtue and vice are not arbitrary things; but there
            is a natural and eternal reason for that goodness
            and virtue, and against vice and wickedness.
                                                  --Tillotson.
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   2. The faculty or capacity of the human mind by which it is
      distinguished from the intelligence of the inferior
      animals; the higher as distinguished from the lower
      cognitive faculties, sense, imagination, and memory, and
      in contrast to the feelings and desires. Reason comprises
      conception, judgment, reasoning, and the intuitional
      faculty. Specifically, it is the intuitional faculty, or
      the faculty of first truths, as distinguished from the
      understanding, which is called the discursive or
      ratiocinative faculty.
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            We have no other faculties of perceiving or knowing
            anything divine or human, but by our five senses and
            our reason.                           --P. Browne.
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            In common and popular discourse, reason denotes that
            power by which we distinguish truth from falsehood,
            and right from wrong, and by which we are enabled to
            combine means for the attainment of particular ends.
                                                  --Stewart.
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            Reason is used sometimes to express the whole of
            those powers which elevate man above the brutes, and
            constitute his rational nature, more especially,
            perhaps, his intellectual powers; sometimes to
            express the power of deduction or argumentation.
                                                  --Stewart.
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            By the pure reason I mean the power by which we
            become possessed of principles.       --Coleridge.
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            The sense perceives; the understanding, in its own
            peculiar operation, conceives; the reason, or
            rationalized understanding, comprehends.
                                                  --Coleridge.
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   3. Due exercise of the reasoning faculty; accordance with, or
      that which is accordant with and ratified by, the mind
      rightly exercised; right intellectual judgment; clear and
      fair deductions from true principles; that which is
      dictated or supported by the common sense of mankind;
      right conduct; right; propriety; justice.
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            I was promised, on a time,
            To have reason for my rhyme.          --Spenser.
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            But law in a free nation hath been ever public
            reason; the enacted reason of a parliament, which he
            denying to enact, denies to govern us by that which
            ought to be our law; interposing his own private
            reason, which to us is no law.        --Milton.
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            The most probable way of bringing France to reason
            would be by the making an attempt on the Spanish
            West Indies.                          --Addison.
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   4. (Math.) Ratio; proportion. [Obs.] --Barrow.
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   By reason of, by means of; on account of; because of.
      "Spain is thin sown of people, partly by reason of the
      sterility of the soil." --Bacon.

   In reason,

   In all reason, in justice; with rational ground; in a right
      view.
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            When anything is proved by as good arguments as a
            thing of that kind is capable of, we ought not, in
            reason, to doubt of its existence.    --Tillotson.
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   It is reason, it is reasonable; it is right. [Obs.]
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            Yet it were great reason, that those that have
            children should have greatest care of future times.
                                                  --Bacon.
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   Syn: Motive; argument; ground; consideration; principle;
        sake; account; object; purpose; design. See Motive,
        Sense.
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