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. To use the faculty of reason; think logically. To talk or argue logically and persuasively. Obsolete To engage in conversation or discussion. To determine or conclude by logical thinking: reasoned out a solution to the problem. To persuade or dissuade (someone) with reasons. 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.