rea·son 
(rēzən)
(rēzən)n.
1.
a. The basis or motive for an action, decision, or conviction: There are good reasons to learn a foreign language. See Usage Notes at because, why.
b. A declaration made to explain or justify action, decision, or conviction: What reasons did she give for leaving?
c. A fact or cause that explains why something exists or has occurred: The reason for the building's collapse is unknown.
d. Logic A premise, usually the minor premise, of an argument.
2.
a. The capacity for logical, rational, and analytic thought; intelligence: "Most of us would like to believe that when we say something is right or wrong, we are using our powers of reason alone" (Carl Zimmer).
b. The limit of what is reasonable: "It is a curious thing that, when a man hates or loves beyond reason, he is ready to go beyond reason to gratify his feelings" (Rudyard Kipling).
c. A normal mental state; sanity: He has lost his reason.
v. rea·soned, rea·son·ing, rea·sons
v.tr.
1. To determine or conclude by logical thinking: The doctor reasoned that the patient had a virus.
2. To persuade or dissuade (someone) with reasons: "You boast ... of having reasoned him out of his absurd romance" (William Makepeace Thackeray).
v.intr.
Idioms: 1. To use the faculty of reason; think logically: What would lead you to reason so?
2. To talk or argue logically and persuasively: tried to reason with her son to eat a good breakfast.
3. Obsolete To engage in conversation or discussion.
by reason of
Because of.
in reason
With good sense or justification; reasonably.
within reason
Within the bounds of good sense or practicality.
with reason
With good cause; justifiably.
[Middle English resoun, from Old French raison, from Latin ratiō, ratiōn-, from ratus, past participle of rērī, to consider, think; see ar- in the Appendix of Indo-European roots.]
reason·er n.
Synonyms: reason, intuition, understanding, judgment
These nouns refer to the intellectual faculty by which humans seek or attain knowledge or truth. Reason is the power to think rationally and logically and to draw inferences: "Mere reason is insufficient to convince us of its [the Christian religion's] veracity" (David Hume).
Intuition is perception or comprehension, as of truths or facts, without the use of the rational process: I trust my intuitions when it comes to assessing someone's character. Understanding is the faculty by which one understands, often together with the resulting comprehension: "The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding" (Louis D. Brandeis).
Judgment is the ability to assess situations or circumstances and draw sound conclusions: "At twenty years of age, the will reigns; at thirty, the wit; and at forty, the judgment" (Benjamin Franklin). See Also Synonyms at cause, think.
These nouns refer to the intellectual faculty by which humans seek or attain knowledge or truth. Reason is the power to think rationally and logically and to draw inferences: "Mere reason is insufficient to convince us of its [the Christian religion's] veracity" (David Hume).
Intuition is perception or comprehension, as of truths or facts, without the use of the rational process: I trust my intuitions when it comes to assessing someone's character. Understanding is the faculty by which one understands, often together with the resulting comprehension: "The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding" (Louis D. Brandeis).
Judgment is the ability to assess situations or circumstances and draw sound conclusions: "At twenty years of age, the will reigns; at thirty, the wit; and at forty, the judgment" (Benjamin Franklin). See Also Synonyms at cause, think.