Wisdom from the Early Stoics, Zeno of Citium 80

Another tenet of the Stoics is the perpetual exercise of virtue, as held by Cleanthes and his followers. For virtue can never be lost, and the good man is always exercising his mind, which is perfect. 

Again, they say that justice, as well as law and right reason, exists by nature and not by convention: so Chrysippus in his work On the Morally Beautiful. 

Neither do they think that the divergence of opinion between philosophers is any reason for abandoning the study of philosophy, since at that rate we should have to give up life altogether: so Posidonius in his Exhortations. 

Chrysippus allows that the ordinary Greek education is serviceable. 

—Diogenes Laërtius, 7.128-129