Merriam-Webster Collegiate Dictionary - reason

 I. noun  Etymology: Middle English resoun, from Anglo-French raisun, from Latin ration-, ratio ~, computation, from reri to calculate, think; probably akin to Gothic rathjo account, explanation  Date: 13th century  1.  a. a statement offered in explanation or justification gave ~s that were quite satisfactory  b. a rational ground or motive a good ~ to act soon  c. a sufficient ground of explanation or of logical defense; especially something (as a principle or law) that supports a conclusion or explains a fact the ~s behind her client's action  d. the thing that makes some fact intelligible ; cause the ~ for earthquakes the real ~ why he wanted me to stay — Graham Greene  2.  a.  (1) the power of comprehending, inferring, or thinking especially in orderly rational ways ; intelligence  (2) proper exercise of the mind  (3) sanity  b. the sum of the intellectual powers  3. archaic treatment that affords satisfaction  II. verb  (~ed; ~ing)  Date: 15th century  intransitive verb  1.  a. obsolete to take part in conversation, discussion, or argument  b. to talk with another so as to influence actions or opinions can't ~ with them  2. to use the faculty of ~ so as to arrive at conclusions  transitive verb  1. archaic to justify or support with ~s  2. to persuade or influence by the use of ~  3. to discover, formulate, or conclude by the use of ~ a carefully ~ed analysis  Synonyms: see think  • ~er noun