reason

To reason is to think things through, to use logic, or to try to solve a problem with logic.

(verb)

An example of reason is when you think though a problem logically.

Reason is the cause for something to happen or the power of your brain to think, understand and engage in logical thought.

(noun)

See reason in Webster's New World College Dictionary

noun

Origin: ME reisun < OFr < L ratio, a reckoning, reason: see read

intransitive verb

to think coherently and logically; draw inferences or conclusions from facts known or assumed
to argue or talk in a logical way

transitive verb

to think logically about; think out systematically; analyze
to argue, conclude, or infer: now usually with a clause introduced by that as the object
to support, justify, etc. with reasons
to persuade or bring by reasoning (into or out of)

See reason in American Heritage Dictionary 4

noun

The basis or motive for an action, decision, or conviction. See Usage Notes at because, why.
A declaration made to explain or justify action, decision, or conviction: inquired about her reason for leaving.
An underlying fact or cause that provides logical sense for a premise or occurrence: There is reason to believe that the accused did not commit this crime.
The capacity for logical, rational, and analytic thought; intelligence.
Good judgment; sound sense.
A normal mental state; sanity: He has lost his reason.
Logic A premise, usually the minor premise, of an argument.