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.

the mental powers concerned with forming conclusions, judgments, or inferences.

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 fol. 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 or 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.