Merriam-Webster's Collegiate® Dictionary, 11th Edition
REASON
I. noun
Etymology: Middle English resoun, from Anglo-French raisun, from Latin ration-, ratio reason, computation, from reri to calculate, think; probably akin to Gothic rathjo account, explanation
Date: 13th century
1. a statement offered in explanation or justification , a rational ground or motive , a sufficient ground of explanation or of logical defense, the thing that makes some fact intelligible ; cause ,
2. a. the power of comprehending, inferring, or thinking especially in orderly rational ways ; intelligence , proper exercise of the mind, sanity , the sum of the intellectual powers, treatment that affords satisfaction,
II. verb (reasoned; reasoning)
Date: 15th century
intransitive verb
1. to take part in conversation, discussion, or argument, to talk with another so as to influence actions or opinions , to use the faculty of reason so as to arrive at conclusions, transitive verb to justify or support with reasons, to persuade or influence by the use of reason, to discover, formulate, or conclude by the use of reason , see: think reasoner noun