Definition of reason:
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; the mind, the reasoning processes of the mind
"he had no reason to do that."
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; 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
"I reasoned the matter with my friend."