What makes this observation sharp rather than sentimental is that Marcus Aurelius isn't merely saying *think positive thoughts*—he's suggesting that repetitive thinking actually alters your character at the deepest level, the way fabric takes on dye irreversibly. A person who habitually rehearses resentment doesn't just feel bitter in the moment; they're slowly becoming a bitter person, their very nature stained. You see this plainly in how people who spend years catastrophizing about health problems often develop genuine anxiety disorders, or how those who regularly indulge grievances can no longer recognize their own capacity for forgiveness. The warning cuts both directions: your mind is not a separate chamber from your soul, but rather its workshop.