Letters 95.45

Marcus Brutus, in the book which he has entitled Concerning Duty, gives many precepts to parents, children, and brothers; but no one will do his duty as he ought, unless he has some principle to which he may refer his conduct. We must set before our eyes the goal of the Supreme Good, towards which we may strive, and to which all our acts and words may have reference—just as sailors must guide their course according to a certain star.

Seneca, Letters to Lucilius 95.45, translated by Richard Mott Gummere