Letters 95.63

Furthermore, when we advise a man to regard his friends as highly as himself, to reflect that an enemy may become a friend, to stimulate love in the friend, and to check hatred in the enemy, we add: “This is just and honourable.” Now the just and honourable element in our doctrines is embraced by reason; hence reason is necessary; for without it the doctrines cannot exist, either.

Seneca, Letters to Lucilius 95.63, translated by Richard Mott Gummere