|Nero: mercenary, despotic
If one who does not recognize the contents of the universe is a stranger in it, no less a stranger is the one who fails to recognize what happens in it. He is a fugitive if he runs away from social principle; blind, if he shuts the eye of the mind; a beggar, if he depends on others and does not possess within him all he needs for life; a tumour on the universe, if he stands aside and separates himself from the principle of our common nature in disaffection with his lot (for it is nature which brings this about, just as it brought you about too); a social splinter, if he splits his own soul away from the soul of all rational beings, which is a unity.
In verse 28, Marcus reminds himself of his potential to be a tyrant Emperor - this was his way of hedging himself from his powerful and swift capacity to make others' life living hell.
Looking within yourself, what dark qualities could you become if you don't yield to reason? This negative visualization could be useful in hedging yourself from a life of vanity.
In verse 29, he further reminds himself of his social duties with regard to others. We live in a social order and any attempt to separate ourselves from that social order goes against our design and our purpose. Engage with others; make a difference; use reason and fulfill your duties.