Letters 94.45

Virtue is divided into two parts—into contemplation of truth, and conduct. Training teaches contemplation, and admonition teaches conduct. And right conduct both practises and reveals virtue. But if, when a man is about to act, he is helped by advice, he is also helped by admonition. Therefore, if right conduct is necessary to virtue, and if, moreover, admonition makes clear right conduct, then admonition also is an indispensable thing.

Seneca, Letters to Lucilius 94.45, translated by Richard Mott Gummere