One of the greatest texts about happiness and living well wasn't written by a self-help expert, spiritual leader or psychologist. It was written by Roman emperor Marcus Aurelius, and it may completely change your perspective on dealing with life's challenges.

In 167 AD, Aurelius wrote The Meditations, a 12-book compendium of personal writings, originally written in Greek, that reflect his extensive study of Stoic philosophy. Aurelius is now regarded as one of the most famous proponents and philosophers of Stoicism, an ancient Greek and Roman school of thought originating in the Hellenic period concerned with how to cultivate a mindset to deal effectively with any events or emotions.

Meditations is based around a single, simple precept: “You have power over your mind -- not outside events. Realize this, and you will find strength.”

The last of the Five Good Emperors, Aurelius ruled over Rome for 20 years until the time of his death in 180 AD. He is widely regarded as one of the most respected emperors in Roman history.

"Marcus Aurelius was a true paradox -- an emperor with almost unlimited power to control his world and circumstances, who nevertheless had a deep understanding that happiness and peace do not lie in the outside world," Arianna Huffington writes in her forthcoming book, Thrive: The Third Metric To Redefining Success And Creating A Life Of Well-Being, Wisdom And Wonder.

Meditations is "undoubtedly one of history's most effective formulas for overcoming every negative situation we may encounter in life," Ryan Holiday writes in The Obstacle Is The Way.

Never got a chance to read Meditations? Here are five of the most important takeaways from the Roman Emperor's magnum opus.

Your own happiness is up to you.

Life's happiness, Aurelius said, "depends upon the quality of your thoughts.”

The crux of his philosophy is the notion that while we cannot control what happens to us, we can control our reactions to the events of our lives -- and this gives us immense strength and freedom.

It's easier said than done, yes, but Aurelius's own life is proof positive of this maxim. The emperor faced great struggles throughout his life, and his reign was marred by near-constant warfare and disease. His brother and parents also died at a young age.

Aurelius learned how to live within his soul -- or "inner citadel," as he put it -- a place of peace and equanimity. Living from this space, he believed, gave him the freedom to shape his own life by controlling his thoughts.

Life may not give you what you want, but it will give you what you need.

Aurelius accepted that trials and challenges were an unavoidable part of life, but his belief that life and the universe were fundamentally good helped him to accept the tough stuff. The argument goes like this: Because life as a whole is as good as it can be, the parts of life are as good as they can be, so we should love, or at least accept, every part of life.

But Aurelius took it even one step further, arguing that obstacles are actually our greatest opportunities for growth and advancement. They force us to re-examine our path, find a new way, and ultimately empower ourselves by practicing virtues like patience, generosity and courage.

"The impediment to action advances action," he wrote. "What stands in the way becomes the way."

There is good in everyone.

Aurelius isn't expressing blind optimism when he advises his readers to find common ground with others and seek the good in every person they encounter. In politics and life, Aurelius had experienced how people could be selfish and hurtful to others -- he lived through wars and uprisings -- and yet, he chose not to let the actions of others get to him. Instead, he always remembered that there is some of the "divine" in each of us:

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 or birth, but the same mind, and possessing a share of the divine. And so none of them can hurt me.

Aurelius believed that all men are made to cooperate with one another, like the "rows of the upper and lower teeth."

True peace comes from within.

Many of us live frantic, high-octane lives -- and we may fantasize about getting away from it all by going on a meditation retreat or taking time off from work to travel. But, as Aurelius strongly believed, you don't need to escape your environment to find a sense of calm. We can access serenity any time in our own minds.

"People look for retreats for themselves, in the country, by the coast, or in the hills," Aurelius wrote. "There is nowhere that a man can find a more peaceful and trouble-free retreat than in his own mind ... So constantly give yourself this retreat, and renew yourself."

Taking a "mental retreat" through a meditation practice -- or simply by bringing more mindfulness into your day -- has been linked to mental health benefits. Meditation has been shown to improve memory and attention, lower stress levels, enhance emotional well-being and sleep quality and boost creativity and productivity.

Treat life as an "old and faithful friend."

Perhaps the most memorable passage of Meditations encourages us to view life as being, in the words of the poet Rumi, "rigged in [our] favor." It's a powerful way of reframing any obstacle we encounter. Aurelius wrote:

True understanding is to see the events of life in this way: 'You are here for my benefit, though rumor paints you otherwise.' And everything is turned to one's advantage when he greets a situation like this: You are the very thing I was looking for. Truly whatever arises in life is the right material to bring about your growth and the growth of those around you. This, in a word, is art -- and this art called 'life' is a practice suitable to both men and gods. Everything contains some special purpose and a hidden blessing; what then could be strange or arduous when all of life is here to greet you like an old and faithful friend?