This is one of the more important passages from Meditations, in my opinion. In it, Marcus marks important boundaries in the human mental model. He breaks down the human into three parts: our bones and muscles and blood and water compose the body; our lungs supply us with the air we need and then our mind allows us to think and act.
We have a responsibility to take care of our body and breath, but they are not entirely in your control. External things and events can easily and quickly take away our breath and body. Our physical brain falls under the domain of the body. But to the extent that the body and brain "work", then how we use our mind is entirely up to us - we have ultimate control over our mind.
In the domain of our mind and thoughts, we get to control what bothers us, what we say, how we act. And everything else outside of this domain (the past, the future, what others do, events, our health and our possessions) is out of our control. If we all could remember that delineation - that border - all the time, we would make great progress on the pathway to becoming a sage. All things become a swirling vortex around our singular mind, and in the midst of it all, our directing mind can "exist on its own." It becomes detached and liberated to think clearly and justly and lovingly embraces all that happens around it.
Marcus then references Empedocles' Sphairos - which is a perfectly round sphere, untouched by external things and events and it is perfectly content and rejoices in its own existence. As he describes this, I can't help but think of an eye of a hurricane - where all around that eye is a ravaging storm of destruction and in the center is perfect calm. To reach that state of mind, in my opinions, is the perfect resilience - the perfect state of mind - to live in the present; to live calmly and kindly and to be at perfect peace with yourself.
(see also Citadel p. 55, 112-113, 134-135, 237, 265)