Philosophical Literature
Marcus's Meditations are not meant to be an original work of philosophy. What we actually get is Marcus himself working through philosophical principles as they pertain to the life of a philosopher-emperor.
Marcus is drawn to Stoicism, which holds as its highest ideals life in harmony with Nature (or reason) and an indifference to both the pleasure and pain that move human beings away from virtues (like courage, self-sufficiency, intelligence, self control, justice).
In the Meditations, you'll see Marcus commanding himself to make a fortress of his mind so that he might always be motivated by philosophical principles rather than less noble emotional responses.