Gandhi isn't simply saying "don't be insulted"—he's drawing a boundary between what others do and what you permit them to do. Notice the active verb: *let*. Your mind remains your property, and allowing someone's negativity, contempt, or carelessness to settle there is a choice you make, not something inflicted upon you. When a colleague's casual cruelty still bothers you three days later, you've essentially invited them to stay; the antidote isn't thicker skin but rather the quiet refusal to grant them that real estate in your thoughts.