Many have suggested various past monarchs embody Plato's philosopher king ideal.

Marcus Aurelius, the Roman Emperor, is often cited as a prime example, though some criticize him for falling short for various reasons.

But Marcus is worth a closer examination because "Meditations," his personal journal, still survives.

In "The Inner Citadel," French philosopher and historian (of philosophy) Pierre Hadot, explains how the journal was used to carry out what he describes as "spiritual exercises." Most of these fit the bill of Stoic philosophical exercises, but Marcus never refers to himself as a Stoic (though he mentions his Stoic teachers). He says that he is practicing philosophy.

Marcus latched on to philosophy as a young man and studied with the era's greatest teachers. Long after he'd become the most powerful man in the western world and gained a reputation among his people as a wise philosopher in his own right, he never gave up seeking out wise philosophers to learn from, even when this would have put him in a "subservient" position.

According to Philostratus, a Greek sophist of the Roman imperial period who was a boy during Marcus's reign, when Marcus was an old man, he was once met on the road walking to the house of a Roman philosopher, and asked where he was going. He replied, "I am now on my way to Sextus the philosopher to learn what I do not yet know."

Meditations was written over a number of years during some of the hardest periods of Marcus's life, and Meditations shows him using philosophy to keep his head on straight.

This entry displays a form of premeditatio malorum, or the premeditation of bad things/evil, which you can find suggested as an exercise by other Stoics, such as Seneca the Younger.

“When you wake up in the morning, tell yourself: the people I deal with today will be meddling, ungrateful, arrogant, dishonest, jealous and surly. They are like this because they can't tell good from evil. But I have seen the beauty of good, and the ugliness of evil, and have recognized that the wrongdoer has a nature related to my own - not of the same blood and birth, but the same mind, and possessing a share of the divine. And so none of them can hurt me. No one can implicate me in ugliness. Nor can I feel angry at my relative, or hate him. We were born to work together like feet, hands and eyes, like the two rows of teeth, upper and lower. To obstruct each other is unnatural. To feel anger at someone, to turn your back on him: these are unnatural.”

We know that Marcus read Plato (he mentions him in his writing), and was certainly aware of the philosopher king ideal. But Marcus repeatedly poo poos the idea that he lives up to such a lofty standard and reminds himself of his flaws. He tells himself to do the right thing as best he can.

"Do what nature demands. Get a move on—if you have it in you—and don’t worry whether anyone will give you credit for it. And don’t go expecting Plato’s Republic; be satisfied with even the smallest progress, and treat the outcome of it all as unimportant."

Marcus was clearly dedicated to philosophy and living according to the four (Stoic) virtues. So he's certainly a reasonable contender for philosopher king, regardless of what people think about his actual reign.