reason

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The basis or motive for an action, decision, or conviction.
• noun A declaration made to explain or justify action, decision, or conviction.
• noun A fact or cause that explains why something exists or has occurred.
• noun Logic A premise, usually the minor premise, of an argument.
• noun The capacity for logical, rational, and analytic thought; intelligence.
• noun Good judgment; sound sense.
• noun A normal mental state; sanity.
• intransitive verb To determine or conclude by logical thinking.
• intransitive verb To persuade or dissuade (someone) with reasons.
• intransitive verb To use the faculty of reason; think logically.
• intransitive verb To talk or argue logically and persuasively.
• intransitive verb Obsolete To engage in conversation or discussion.
• idiom (by reason of) Because of.
• idiom (in reason) With good sense or justification; reasonably.
• idiom (within reason) Within the bounds of good sense or practicality.
• idiom (with reason) With good cause; justifiably.

from The Century Dictionary.

• noun An obsolete spelling of raisin. In the following passage it is apparently applied to some other fruit than the grape.
• To exereise the faculty of reason; make rational deductions; think or choose rationally; use intelligent discrimination.
• To practise reasoning in regard to something; make deductions from premises; engage in discussion; argue, or hold arguments.
• To hold account; make a reckoning; reckon.
• To hold discourse; talk; parley.
• To reason about; consider or discuss argumentatively; argue; debate.
• To give reasons for; support by argument; make a plea for: often with out: as, to reason out a proposition or a claim.
• To persuade by reasoning or argument.
• To hold argument with; engage in speech or discussion; talk with; interrogate.
• noun An idea acting as a cause to create or confirm a belief, or to induce a voluntary action; a judgment or belief going to determine a given belief or line of conduct.
• noun A fact, known or supposed, from which another fact follows logically, as in consequence of some known law of nature or the general course of things; an explanation.
• noun An intellectual faculty, or such faculties collectively.
• noun The logical faculties generally, including all that is subservient to distinguishing truth and falsehood, except sense, imagination, and memory on the one hand, and the faculty of intuitively perceiving first principles, and other lofty faculties, on the other.
• noun The faculty of drawing conclusions or inferences, or of reasoning.
• noun The faculty by which we attain the knowledge of first principles; a faculty for apprehending the unconditioned.
• noun Intelligence considered as having universal validity or a catholic character, so that it is not something that belongs to any person, but is something partaken of, a sort of light in which every mind must perceive.
• noun That which recommends itself to enlightened intelligence; some inward intimation for which great respect is felt and which is supposed to be common to the mass of mankind; reasonable measure; moderation; right; what mature and cool reflection, taking into account the highest considerations, pronounces for, as opposed to the prompting of passion.
• noun A reasonable thing; a rational thing to do; an idea or a statement conformable to common sense.
• noun The exercise of reason; reasoning; right reasoning; argumentation; discussion.
• noun The intelligible essence of a thing or species; the quiddity.
• noun In logic, the premise or premises of an argument, especially the minor premise.
• noun By right or justice; properly; justly.
• noun In French history, an act of worship of human reason, represented by a woman as the goddess of Reason, performed on November 10th, 1793, in the cathedral of Notre Dame, and also in other churches (renamed temples of Rea son) in France on that and succeeding days. The worship of Reason was designed to take the place of the suppressed Christian worship; recognition of the Supreme Being was restored through the influence of Robespierre.
• noun Agreeable to reason; reasonable; just; proper; as, I will do anything in reason.
• noun The human understanding; the discursive reason.
• noun See do.
• noun Synonyms Inducement, etc. (see motive), account, object, purpose, design.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb 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.
• intransitive verb 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.
• intransitive verb To converse; to compare opinions.
• noun 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.
• noun 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.