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Marcus Aurelius was a Roman emperor and a stoic philosopher. (TV: Deep Breath [+]Loading...["Deep Breath (TV story)"])
According to Clara Oswald, Aurelius was known as the last of the "five good 'uns". Vastra claimed he was a "[s]uperlative bass guitarist," and mentioned the Doctor knew how to put a band together, though she did not elaborate further as to the Doctor's relationship with him. It's also unclear whether Vastra meant this literally or as a metaphor for some other relationship with the Doctor.
Clara Oswald was a fan of the thinker, and had a poster of his in her bedroom when she was 15. When being criticised by Vastra over her negative response to the Twelfth Doctor's change to an older appearance, in which Vastra accused Clara of ageism, Clara used this fact to show that a man's apparent age played no role in whether she found him attractive or not. (TV: Deep Breath [+]Loading...["Deep Breath (TV story)"])
She once taught a class at Coal Hill School in which she quoted him as saying, "Waste no more time arguing about what a good man should be. Be one." (TV: The Day of the Doctor [+]Loading...["The Day of the Doctor (TV story)"]) This axiom was also at the core of some of her early interactions with the Twelfth Doctor in which he questioned whether he was a "good man". (TV: Into the Dalek [+]Loading...["Into the Dalek (TV story)"])