Iron rusts from disuse; stagnant water loses it purity and in cold weather becomes frozen; even so does inaction sap the vigor of the mind.

The quote argues that mental strength is not a fixed possession but a capacity maintained through activity. By comparing the mind to iron that rusts and water that spoils when left still, it frames inaction as a kind of decay: neglect produces deterioration even without overt harm. The final clause—“inaction sap[s] the vigor of the mind”—suggests that intellectual vitality depends on practice, curiosity, and sustained engagement, not merely innate talent. In a broader sense, it endorses habits of continual learning and making, implying that creativity and judgment are preserved by regular exercise and are threatened by passivity, comfort, or procrastination.