Meditations 8:41

For animate beings, "harmful" is whatever obstructs the operation of their senses—or the fulfillment of what they intend. Similar obstructions constitute harm to plants. So too for rational creatures, anything that obstructs the operation of the mind is harmful.

Apply this to yourself.

Do pain and pleasure have their hooks in you? Let the senses deal with it. Are there obstacles to your action? If you failed to reckon with the possibility, then that would harm you, as a rational being. But if you use common sense, you haven't been harmed or even obstructed. No one can obstruct the operations of the mind. Nothing can get at them—not fire or steel, not tyrants, not abuse—nothing. As long as it's "a sphere . . . in perfect stillness."