Those who know me know I have read a lot of books on leadership over the years. Some were excellent. Some offered ideas that shaped how I approached my career. Others, frankly, I have long since forgotten.

But one book I keep returning to.

It wasn't written by a CEO, management consultant, business school professor or leadership guru. A Roman emperor wrote it nearly 2,000 years ago: Marcus Aurelius.

If I had my way, Meditations would be required reading for every leader.

Why? Because Marcus Aurelius understood something about leadership that we sometimes make far too complicated: Before you can effectively lead others, you must first learn to lead yourself.

Marcus Aurelius ruled Rome from 161 to 180 AD. He is remembered as one of its more benevolent emperors, but his life was hardly one of uninterrupted privilege. He lost several of his children. He confronted military threats along the empire's borders and endured the devastating Antonine Plague. A trusted general betrayed him and attempted to overthrow him. Even his personal life was difficult.

How did he cope?

He wrote.

Often while traveling with his armies and confronting enemies along Rome's borders, Marcus Aurelius recorded his thoughts about what it meant to be a good man and a responsible leader. As a Stoic, he reminded himself to remain disciplined in the face of difficult people and circumstances, to distinguish between what he could control and what he could not, and to focus on the quality of his own response.

He wasn't writing for an audience. These were private reflections, essentially a journal through which he challenged himself to become better. Those writings survived and eventually became known as Meditations.

Nearly two millennia later, the lessons remain remarkably relevant.

Leadership hasn't changed nearly as much as we sometimes think. Leaders still make decisions without complete information. We still face criticism, difficult personalities, unexpected crises and circumstances entirely beyond our control. We still contend with ego, frustration, uncertainty and bad days.

What we can control is how we respond. We can control our attitude, our effort, our judgment, our character and, perhaps most importantly, how we treat people when the pressure is on.

During my leadership career, I learned that people watch their leaders most closely when things aren't going well. When everyone else is anxious, they look to you. When the plan isn't working, they look to you. When something has gone terribly wrong, they look to you.

I always believed my responsibility in those moments was to be the calm center. Listen first. Understand the situation. Determine the strategy. Then move forward.

That is easier said than done, of course. Which is why another of Marcus Aurelius' lessons is so important: Keep your ego in check.

Consider the humility required for the most powerful man in the known world to repeatedly remind himself not to become impressed with his own power. He understood that position, authority and status were temporary. Character was not.

Modern leaders could use more of that perspective. I certainly could have during my career.

Because ultimately, leadership is about character.

People may forget the presentation you gave or the strategy document you produced. They may not remember every decision you make. But they will remember how you treated them. They will remember whether you listened, whether you remained composed during a crisis, whether you accepted responsibility when things went wrong and whether you gave others credit when things went right.

Most importantly, they will remember whether the person you claimed to be was the same person they encountered when the pressure arrived.

That may be the enduring power of Meditations. It is the private record of a powerful man who continually challenged himself to be better—to be a better leader, citizen, friend, husband and father.

He wasn't writing to teach others how to lead. He was reminding himself how to live.

Two thousand years later, that distinction may be precisely what makes his lessons so valuable. Leadership theories come and go. Technology changes. Organizations evolve. The challenges facing leaders take new forms.

But the fundamental test of leadership remains remarkably constant.

A lot has changed in 2,000 years. What makes a great leader hasn't.