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. [1913 Webster]
"I'll give him reasons for it." [1913 Webster]
"The reason of the motion of the balance in a wheel watch is by the motion of the next wheel." [1913 Webster]
"This reason did the ancient fathers render, why the church was called “catholic.”" [1913 Webster]
"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." [1913 Webster]

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. [1913 Webster]
"We have no other faculties of perceiving or knowing anything divine or human, but by our five senses and our reason." [1913 Webster]
"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." [1913 Webster]
"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." [1913 Webster]
"By the pure reason I mean the power by which we become possessed of principles." [1913 Webster]
"The sense perceives; the understanding, in its own peculiar operation, conceives; the reason, or rationalized understanding, comprehends." [1913 Webster]

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. [1913 Webster]
"I was promised, on a time,
To have reason for my rhyme." [1913 Webster]
"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." [1913 Webster]
"The most probable way of bringing France to reason would be by the making an attempt on the Spanish West Indies." [1913 Webster]

4.  Ratio; proportion. Barrow. [1913 Webster]
"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." [1913 Webster]
"Yet it were great reason, that those that have children should have greatest care of future times." [1913 Webster]

1.  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. [1913 Webster]

2.  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. [1913 Webster]
"Stand still, that I may reason with you, before the Lord, of all the righteous acts of the Lord." [1913 Webster]

3.  To converse; to compare opinions. Shak. [1913 Webster]

1.  To arrange and present the reasons for or against; to examine or discuss by arguments; to debate or discuss; as, I reasoned the matter with my friend. [1913 Webster]
"When they are clearly discovered, well digested, and well reasoned in every part, there is beauty in such a theory." [1913 Webster]

2.  To support with reasons, as a request. Shak. [1913 Webster]

3.  To persuade by reasoning or argument; as, to reason one into a belief; to reason one out of his plan. [1913 Webster]
"Men that will not be reasoned into their senses." [1913 Webster]

4.  To overcome or conquer by adducing reasons; -- with down; as, to reason down a passion. [1913 Webster]

5.  To find by logical processes; to explain or justify by reason or argument; -- usually with out; as, to reason out the causes of the librations of the moon. [1913 Webster]

reason, n. & v.
--n.
1 a motive, cause, or justification (has good reasons for doing this; there is no reason to be angry).
2 a fact adduced or serving as this (I can give you my reasons).
3 the intellectual faculty by which conclusions are drawn from premisses.
4 sanity (has lost his reason).
5 Logic a premiss of a syllogism, esp. a minor premiss when given after the conclusion.
6 a faculty transcending the understanding and providing a priori principles; intuition.
7 sense; sensible conduct; what is right or practical or practicable; moderation.
--v.
1 intr. form or try to reach conclusions by connected thought.
2 intr. (foll. by with) use an argument (with a person) by way of persuasion.
3 tr. (foll. by that + clause) conclude or assert in argument.
4 tr. (foll. by why, whether, what + clause) discuss; ask oneself.
5 tr. (foll. by into, out of) persuade or move by argument (I reasoned them out of their fears).
6 tr. (foll. by out) think or work out (consequences etc.).
7 tr. (often as reasoned adj.) express in logical or argumentative form.
8 tr. embody reason in (an amendment etc.).

Idiom:
by reason of owing to. in (or within) reason within the bounds of sense or moderation. it stands to reason (often foll. by that + clause) it is evident or logical. listen to reason be persuaded to act sensibly. see reason acknowledge the force of an argument. with reason justifiably.

Derivative:
reasoner n. reasoning n. reasonless adj.

Etymology:
ME f. OF reisun, res(o)un, raisoner, ult. f. L ratio -onis f. reri rat- consider

N  motive, springs of action, wellsprings of action, reason, ground, call, principle, by end, by purpose, mainspring, primum mobile, keystone, the why and the wherefore, pro and con, reason why, secret motive, arriere pensee, intention, inducement, consideration, attraction, loadstone, magnet, magnetism, magnetic force, allectation, allective, temptation, enticement, agacerie, allurement, witchery, bewitchment, bewitchery, charm, spell, fascination, blandishment, cajolery, seduction, seducement, honeyed words, voice of the tempter, song of the Sirens forbidden fruit, golden apple, persuasibility, persuasibleness, attractability, impressibility, susceptibility, softness, persuasiveness, attractiveness, tantalization, influence, prompting, dictate, instance, impulse, impulsion, incitement, incitation, press, instigation, provocation, inspiration, persuasion, suasion, encouragement, advocacy, exhortation, advice, solicitation, lobbyism, pull, incentive, stimulus, spur, fillip, whip, goad, ankus, rowel, provocative, whet, dram, bribe, lure, decoy, decoy duck, bait, trail of a red herring, bribery and corruption, sop, sop for Cerberus, prompter, tempter, seducer, seductor, instigator, firebrand, incendiary, Siren, Circe, agent provocateur, lobbyist, impulsive, motive, suasive, suasory, persuasive, persuasory, hortative, hortatory, protreptical, inviting, tempting suasive, suasory, seductive, attractive, fascinating, provocative, induced, disposed, persuadable, spellbound, instinct with, smitten with, infatuated, inspired, by, because, therefore, from this motive, from that motive, for this reason, for that reason, for, by reason of, for the sake of, count of, out of, from, as, forasmuch as, for all the world, on principle, fax mentis incendium gloriae, temptation hath a music for all ears, to beguile many and be beguiled by one.

N  cause, origin, source, principle, element, occasioner, prime mover, primum mobile, vera causa, author, mainspring, agent, leaven, groundwork, foundation, spring, fountain, well, font, fountainhead, spring head, wellhead, fons et origo, genesis, descent, remote cause, influence, pivot, hinge, turning point, lever, crux, fulcrum, key, proximate cause, causa causans, straw that breaks the camel's back, ground, reason, reason why, why and wherefore, rationale, occasion, derivation, final cause, les dessous des cartes, undercurrents, rudiment, egg, germ, embryo, bud, root, radix radical, etymon, nucleus, seed, stem, stock, stirps, trunk, tap-root, gemmule, radicle, semen, sperm, nest, cradle, nursery, womb, nidus, birthplace, hotbed, causality, causation, origination, production, caused, causal, original, primary, primitive, primordial, aboriginal, protogenal, radical, embryonic, embryotic, in embryo, in ovo, seminal, germinal, at the bottom of, connate, having a common origin, because, behind the scenes, causa latet vis est notissima, felix qui potuit rerum cognoscere causas.

N  intellect, mind, understanding, reason, thinking principle, rationality, cogitative faculties, cognitive faculties, discursive faculties, reasoning faculties, intellectual faculties, faculties, senses, consciousness, observation, percipience, intelligence, intellection, intuition, association of ideas, instinct, conception, judgment, wits, parts, capacity, intellectuality, genius, brains, cognitive powers, intellectual powers, wit, ability, wisdom, Vernunft, Verstand, soul, spirit, ghost, inner man, heart, breast, bosom, penetralia mentis, divina particula aurae, heart's core, the Absolute, psyche, subliminal consciousness, supreme principle, brain, organ of thought, seat of thought, sensorium, sensory, head, headpiece, pate, noddle, noggin, skull, scull, pericranium, cerebrum, cranium, brainpan, sconce, upper story, central processing unit, CPU, arithmetic and logical unit, ALU, metaphysics, psychics, psychology, ideology, mental philosophy, moral philosophy, philosophy of the mind, pneumatology, phrenology, craniology, cranioscopy, ideality, idealism, transcendentalism, spiritualism, immateriality, universal concept, universal conception, metaphysician, psychologist, intellectual, mental, rational, subjective, metaphysical, nooscopic, spiritual, ghostly, psychical, psychological, cerebral, animastic, brainy, hyperphysical, superphysical, subconscious, subliminal, immaterial, endowed with reason, in petto, ens rationis, frons est animi janua, locos y ninos dicen la verdad, mens sola loco non exulat, my mind is my kingdom, stern men with empires in their brains, the mind, the music breathing from her face, thou living ray of intellectual Fire.

VB  reason, argue, discuss, debate, dispute, wrangle, argufy, bandy words, bandy arguments, chop logic, hold an argument, carry on an argument, controvert, canvass, comment upon, moralize upon, spiritualize, consider, open a discussion, open a case, try conclusions, join issue, be at issue, moot, come to the point, stir a question, agitate a question, ventilate a question, torture a question, take up a side, take up a case, contend, take one's stand upon, insist, lay stress on, infer, follow from.

N  intelligence, wisdom, intelligence, capacity, comprehension, understanding, cuteness, sabe, savvy, intellect, nous, parts, sagacity, mother wit, wit, esprit, gumption, quick parts, grasp of intellect, acuteness, acumen, subtlety, penetration, perspicacy, perspicacity, discernment, due sense of, good judgment, discrimination, cunning, refinement, head, brains, headpiece, upper story, long head, eagle eye, eagle- glance, eye of a lynx, eye of a hawk, wisdom, sapience, sense, good sense, common sense, horse sense, plain sense, rationality, reason, reasonableness, judgment, solidity, depth, profundity, caliber, enlarged views, reach of thought, compass of thought, enlargement of mind, genius, inspiration, geist, fire of genius, heaven-born genius, soul, talent, prudence, vigilance, tact, foresight, sobriety, self-possession, aplomb, ballast, a bright thought, not a bad idea, Solomon-like wisdom, intelligent, quick of apprehension, keen, acute, alive, brainy, awake, bright, quick, sharp, quick witted, keen witted, clear witted, sharp-eyed, sharp sighted, sharp witted, wide- awake, canny, shrewd, astute, clear-headed, farsighted, discerning, perspicacious, penetrating, piercing, argute, quick-witted, nimble-witted, needle-witted, sharp as a needle, sharp as a tack, alive to, clever, arch, pas si bete, acute, wise, sage, sapient, sagacious, reasonable, rational, sound, in one's right mind, sensible, abnormis sapiens, judicious, strong- minded, unprejudiced, unbiased, unbigoted, unprepossessed, undazzled, unperplexed, unwarped judgment, impartial, equitable, fair, cool, cool-headed, long-headed, hardheaded, strong-headed, long- sighted, calculating, thoughtful, reflecting, solid, deep, profound, oracular, heaven-directed, heaven-born, prudent, sober, stand, solid, considerate, politic, wise in one's generation, watchful, provident, in advance of one' age, wise as a serpent, wise as Solomon, wise as Solon, wise, sensible, reasonable, judicious, well- thought-out, well-planned, well-judged, well-advised, prudent, politic, expedient, aut regem aut fatuum nasci oportet, but with the morning cool reflection came, flosculi sententiarum, les affaires font les hommes, mas vale saber que haber, mas vale ser necio que profiadol nemo solus sapit, nosce te, gnothi seauton, nullum magnum ingenium sine mixtura dementiae fuit, sapere aude, victrix fortunae sapientia.

age of reason, for some reason, reason out, reason to believe, with reason, within reason, without rhyme or reason