1. The basis or motive for an action, decision, or conviction. See Usage Notes at because, why.
2. A declaration made to explain or justify action, decision, or conviction: inquired about her reason for leaving.
3. 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.
4. The capacity for logical, rational, and analytic thought; intelligence.
5. Good judgment; sound sense.
6. A normal mental state; sanity: He has lost his reason.
7. Logic A premise, usually the minor premise, of an argument.

1. To use the faculty of reason; think logically.
2. To talk or argue logically and persuasively.
3. Obsolete To engage in conversation or discussion.

1. To determine or conclude by logical thinking: reasoned out a solution to the problem.
2. To persuade or dissuade (someone) with reasons.

Synonyms: 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, mind, think.