Letters 57.4

For there are certain emotions, my dear Lucilius, which no courage can avoid; nature reminds courage how perishable a thing it is. And so he will contract his brow when the prospect is forbidding, will shudder at sudden apparitions, and will become dizzy when he stands at the edge of a high precipice and looks down. This is not fear; it is a natural feeling which reason cannot rout.

Seneca, Letters to Lucilius 57.4, translated by Richard Mott Gummere