Seneca presents a powerful metaphor that reframes adversity as mental exercise rather than misfortune. Just as physical resistance builds muscle through challenge and stress, mental challenges build character, resilience, and wisdom through difficulty. This doesn't mean seeking unnecessary hardship, but it means approaching inevitable difficulties with the understanding that they're not just problems to endure but opportunities to develop capacities we'll need for future challenges. Each difficulty we navigate successfully increases our confidence and capability for the next one. Like physical training, mental strengthening happens gradually through consistent engagement with appropriate challenges.