reason (cause) / reason (mental faculties) / common difference (of an arithmetic series) / common ratio (of a geometric series)

with good reason, rightly, understandably

(literary) to be reasonable

(transitive with de) to account for, to explain; to justify

(of a person) to be right; to be correct

raison d'être

(politics) a state interest, especially when invoked as politically superior to moral or even legal considerations

(transitive with à) to prove someone/something right, to bear someone/something out / (transitive with à) to agree with

to see sense, to see reason, to listen to reason

age of reason