Harm to you cannot subsist in another's directing mind, nor indeed in any turn or change of circumstance. Where, then? In that part of you which judges harm. So no such judgement, and all is well. Even if what is closest to it, your own body, is subjected to knife or cautery, or left to suppurate or mortify, even so that faculty in you which judges these things should stay untroubled. That is, it should assess nothing either bad or good which can happen equally to the bad man or the good: because what can happen to a man irrespective of his life's conformity to nature is not of itself either in accordance with nature or contrary to it.
To set the stage: you and another person; both of you have your mind and both of you would agree that you cannot reach into their mind and cause harm and vice versa. If you agree with this, then you agree that the other person cannot harm you with their mind. If you disagree, please provide examples and proof of another person's mind harming your mind.
Furthermore, let's use an example. A person says to you, "you are worthless; you suck; you're a loser." Is your mind harmed? I would argue, no, it is not harmed. Perhaps you still disagree. What if the other person who told you those things were a complete stranger off the street? Would you "feel bad?" Most likely not. But if the other person were a close relative, it might sting a bit, but only because in your mind, that other person is important to you and at some level you want their respect and love. In this case, you have added the importance of the words of the other person. The words and thoughts from the other person's mind did not cause you harm, but your judgement about the person are what determined the weight of those words and thoughts. This whole exercise if not adding what ought not to be added, is the discipline of assent. In a more succinct description, it is simply an attitude adjustment.
Marcus applies the same concept to harm to the body. If your body goes under the knife or is cauterized, then the pain from the knife and cauterization is what it is. You don't need to add to the physical pain the mental anguish, "why is this happening to me??" In my estimation, this is easier said than done. But the idea does help. Examples: me being bit by a venomous snake; me receiving a catheter ablation. In both events, physical pain was applied, but I tried hard not to add to the pain by applying self-mental-anguish.
(see also Citadel p. 41)