In the constitution of the rational being I can see no virtue that counters justice: but I do see the counter to pleasure self-control.
If you remove your judgement of anything that seems painful, you yourself stand quite immune to pain. 'What self?' Reason. 'But I am not just reason.' Granted. So let your reason cause itself no pain, and if some other part of you is in trouble, it can form its own judgement for itself.
An obstacle to sense perception is harmful to animal nature. An obstacle to impulse is likewise harmful to animal nature. (Something else will be similarly obstructive and harmful to the constitution of plants.) It follows that an obstacle to the mind is harmful to intelligent nature. Now apply all this to yourself. Is pain or pleasure affecting you? That is for the senses. You have formed an impulse and then met some obstruction? If this was an unconditional aim, then, yes, the obstruction harms your rational nature: but if you accept what is common experience, no harm is yet done or hindrance caused. You see, no one else will impede the proper functions of the mind. The mind cannot be touched by fire, steel, tyranny, slander, or anything whatever, once it has become 'a perfect round in solitude'.
I have no cause to hurt myself: I have never consciously hurt anyone else.
If you are going to judge, then judge well.
There appears to be some debate about what justice is. Kids seem to have a concept of justice in that they want to ensure fairness is enforced. But therein lies the rub: what is fair? Being kind and selfless to others is a start. If you have some candy and are eating it in front of others, it would be kind and just to offer to share it. If you have four children and you give one a birthday present, it's only fair that you give a present to the other three on their birthday too. And the present ought to be special to them and you ought to put some thought into it. Carrying out justice on a micro and macro scale is not an easy task. Often when one person tries to carry out justice, another person thinks it unfair. So what is to be done with exercising the virtue of justice? Learning and talking about it is a start. Debating examples and finding a golden mean is also a worthy exercise. In any case, putting some thought and logic and rationale behind your judgement and action is always a good idea.
From passage 40 in Book 8, Marcus says that judgement about pain makes it so. You can be immune to pain if you change your opinion. This week, on Reddit, I read a quote from Seneca that said something very similar.
"But do not of your own accord make your troubles heavier to bear and burden yourself with complaining. Pain is slight if opinion has added nothing to it; but if, on the other hand, you begin to encourage yourself and say, "It is nothing, – a trifling matter at most; keep a stout heart and it will soon cease"; then in thinking it slight, you will make it slight. Everything depends on opinion; ambition, luxury, greed, hark back to opinion. It is according to opinion that we suffer" -Seneca, Moral LettersIn passage 41, Marcus continues the logic behind how true pain is all opinion. No one or no thing can harm your mind or your opinions. Your mind and the opinion you form are your own and they are in your control. It is a breath of fresh air to think the mind truly cannot be touched by "fire, steel, tyranny, slander or anything whatever, once it has become 'a perfect round in solitude.'" Getting to that point is not so easy, though.
Next, Marcus reminds himself of his duties to others and to himself. No harm needed for either.
(see also Citadel p. 41, 103, 270, 287)