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.

1. Use of reason, especially to form conclusions, inferences, or judgments.
2. Evidence or arguments used in thinking or argumentation.