Although a great Roman emperor, Marcus Aurelius is often overshadowed by the other great Stoic philosophers, Marcus Aurelius was a significant figure in his own right. His most outstanding contribution to Stoicism was his development of the concept of apatheia.

Apatheia is the Stoic ideal of being free from passion or emotion. For Marcus Aurelius, this meant living a life of detachment, reason, and self-control. This philosophy departed from the more traditional Stoic view that saw emotions as something to be conquered. Instead, Marcus Aurelius believed that we could achieve a state of inner peace by learning to control our emotions.

What does apatheia actually mean?
In Stoicism, apatheia (Greek: from a- "without" and pathos "suffering" or "passion") refers to a state of mind that is unaffected by the passions. Rather than indifference, the word equanimity is perhaps the best translation of apatheia

A detachment from the passions and eudaimonia
Marcus Aurelius believed in the power of reason and self-control. He thought that apatheia, or detachment from the passions, was the key to achieving eudaimonia, or human flourishing. 

Marcus Aurelius believed that Stoicism could help people live in harmony with themselves and their world. By learning to control their emotions and desires, stoics could achieve a state of inner peace and contentment.

Seneca and apatheia

The term apatheia is generally credited to the Greek philosopher Seneca. He is believed to have introduced the term apatheia into Stoic philosophy. In Seneca's view, apatheia was a critical virtue that contributed to eudaimonia, or human flourishing. 

Marcus Aurelius' views on apatheia were highly influential in the development of Stoicism as a whole. Moreover, his ideas helped shape how subsequent Stoic philosophers would think about emotions and their role in our lives. Marcus Aurelius' contribution to Stoicism is still evident in philosophy today, and his influence will continue to be felt for many years to come.