Reason

a basis or cause, as for some belief, action, fact, event, etc.:
the reason for declaring war.
a statement presented in justification or explanation of a belief or action:
I dare you to give me one good reason for quitting school!
the mental powers concerned with forming conclusions, judgments, or inferences:
Effective leadership requires a person of reason.
sound judgment; good sense.
normal or sound powers of mind; sanity.
Logic. a premise of an argument.
Philosophy.

the faculty or power of acquiring intellectual knowledge, either by direct understanding of first principles or by argument.
the power of intelligent and dispassionate thought, or of conduct influenced by such thought.
Kantianism. the faculty by which the ideas of pure reason are created.

to think or argue in a logical manner.
to form conclusions, judgments, or inferences from facts or premises.
to urge reasons which should determine belief or action.
to think through logically, as a problem (often followed by out).
to conclude or infer.
to convince, persuade, etc., by reasoning.
to support with reasons.
bring (someone) to reason, to induce a change of opinion in (someone) through presentation of arguments; convince:
The mother tried to bring her rebellious daughter to reason.
by reason of, on account of; because of:
He was consulted about the problem by reason of his long experience.
in / within reason, in accord with reason; justifiable; proper:
She tried to keep her demands in reason.
stand to reason, to be clear, obvious, or logical:
With such an upbringing it stands to reason that the child will be spoiled.
with reason, with justification; properly:
The government is concerned about the latest crisis, and with reason.
Contemporary Examples

DC Appellate Court Rules Obama’s Recess Appointment Unconstitutional Megan McArdle January 24, 2013
Beyoncé’s Sparkling Nipples; Marco Zanini Could Be Heading to Schiaparelli The Fashion Beast Team April 15, 2013
Fact-Checking the Sunday Shows: July 27 PunditFact.com July 26, 2014
John McCain to Meet With Susan Rice to Settle Benghazi Score Eli Lake November 25, 2012
When the GOP Lost Touch with Conservation Justin Green September 26, 2012

Historical Examples

Buried Cities, Part 2 Jennie Hall
The Spenders Harry Leon Wilson
The Middle Class Gentleman Moliere
The Spenders Harry Leon Wilson
Another Sheaf John Galsworthy

noun
the faculty of rational argument, deduction, judgment, etc
sound mind; sanity
a cause or motive, as for a belief, action, etc
an argument in favour of or a justification for something
(philosophy) the intellect regarded as a source of knowledge, as contrasted with experience
(logic) grounds for a belief; a premise of an argument supporting that belief
by reason of, because of
in reason, within reason, within moderate or justifiable bounds
it stands to reason, it is logical or obvious: it stands to reason that he will lose
listen to reason, to be persuaded peaceably
reasons of State, political justifications for an immoral act
verb
(when transitive, takes a clause as object) to think logically or draw (logical conclusions) from facts or premises
(intransitive) usually foll by with. to urge or seek to persuade by reasoning
(transitive) often foll by out. to work out or resolve (a problem) by reasoning
n.
v.
see:

by reason of
in reason
it stands to reason
listen to reason
lose one’s mind (reason)
rhyme or reason
see reason
stand to reason
with reason