Seneca isn't merely saying that hardship makes us tougher—he's drawing a distinction between mind and body that matters: the body grows through repetition of the same stimulus, but the mind seems to require *variety* in difficulty to strengthen. A person who faces the same obstacle repeatedly may simply memorize workarounds, whereas genuine mental development demands novel problems that force us to think in new ways. Consider someone learning only from textbooks versus someone who must troubleshoot unexpected breakdowns in their field—the latter develops something closer to what Seneca means by a strengthened mind, not just accumulated knowledge.