Perfection of character is this: to live each day as if it were your last, without frenzy, without apathy, without pretence.
The true self in an individual's mind has complete control over reacting to circumstances and can remain content in any situation. Indeed, this is hard philosophy to put in practice. Marcus goes even as far as saying that the mind (you) can be content when a wild beast is tearing off your limbs!
No matter what life dishes out to us, we can take it and use it as "raw material" for our reaction and action. Thus, being mentally prepared for anything allows us to take appropriate, rational and social action. And when we take anything that life throws at us and we react and act virtuously, we are living the art of man and God. We can handle anything.
With this mindset, we can now approach each day, each moment as our last in this life. We can react without frenzy, without apathy and without pretense.
(see also Citadel p. 109, 196, 268, 187)