Letters 85.25

Now those who assert this are doubling back to their old argument, in that they regard vices of less degree as equivalent to virtues. For indeed the man who does feel fear, though he feels it rather seldom and to a slight degree, is not free from wickedness, but is merely troubled by it in a milder form. “Not so,” is the reply, “for I hold that a man is mad if he does not fear evils which hang over his head.” What you say is perfectly true, if the things which threaten are really evils; but if he knows that they are not evils and believes that the only evil is baseness, he will be bound to face dangers without anxiety and to despise things which other men cannot help fearing. Or, if it is the characteristic of a fool and a madman not to fear evils, then the wiser a man is the more he will fear such things!

Seneca, Letters to Lucilius 85.25, translated by Richard Mott Gummere