I didn’t truly understand the meaning of courage until I met Senator John McCain. Although we inhabited opposite ends of the political spectrum, I had always admired his fierce endurance and uncompromising loyalty to country in the face of interrogations, beatings, and solitary confinement as a Vietnam POW. Despite the allure of taking the easy way out, McCain refused to become a propaganda tool or give into doubt and despair. Down to a feeble 105 pounds and suffering from dysentery, he embodied mettle to men who wondered if they would ever see their homes again.

On the surface, he was fearless. As a pilot, he threw himself into harm’s way despite knowing the consequences. In politics, he became the maverick, the lone voice who wasn’t afraid to speak up and buck the system. However, I only understood half of the equation of courage. In reality, courage was something more. It was action balanced by fear and faith informed by vulnerability. For McCain, courage was the starting point, the singular and defining value that ultimately determined where you go and who you become. His definition of “courage” became such an epiphany to me.

As the editor-in-chief of Fast Company magazine in 2004, I was putting together an entire issue devoted to courage and its important to leadership. When I thought about courageous people, he was the first person to spring to mind. So I asked him to write our keynote essay on the topic. He graciously accepted the assignment and later appeared at a reception to launch our issue. The essay McCain ultimately turned in one of the most thoughtful and impactful statements on courage I have ever read. In the essay, entitled Why Guts Matter, McCain defined courage in a way that could only come from a man who’d truly ached, questioned, and come to terms with his limitations – but not with the possibilities around him.

“Courage is not the absence of fear,” he wrote, “but the capacity to act despite our fears.”

That quote resonated with me yesterday when I learned that McCain had been diagnosed with a brain tumor. Years ago, I had received a cancer diagnosis. After I received the prognosis, I was stunned and silent, a numbness that quickly gave way to questions and regrets that began racing through my head. Most of all, I felt afraid.

At that moment, you don’t view yourself as courageous. You find yourself a victim, a castaway shunned by the fates. In those moments, you discover, like McCain, what courage really is. Courage isn’t a decisive act, but a series of small and tentative steps, always riddled with awkward uncertainty. “Courage is like a muscle,” McCain wrote. “The more we exercise it, the stronger it gets.”

This regular exercise isn’t easy, especially when you wonder if it will even matter in the end. As McCain learned, the exercise of courage doesn’t cure festering tumors or cancers. Instead, it nurtures spiritual health, kindling the desire to continue loving, laughing, fighting and even fearing. Courage may not always be the polestar lighting your path, but it keeps you from fading into the darkness.

McCain considers courage to be the “enforcing virtue,” “the one that makes possible all the other virtues common to exceptional leaders: honesty, integrity, confidence, compassion, and humility.” Despite his heroism in Vietnam, McCain admits that he has taken shortcuts, times when he “avoided the risk of injury or disappointment by overruling the demands of my conscience.” Like a muscle that has been enflamed or torn, McCain’s courage would eventually return because of the ongoing lesson that only haunts those who consciously strive to learn from their weakest moments.

“In the past, I’ve been able to overcome my own fears because of an acute sense of an even greater fear — that of feeling remorse,” he writes. “You can live with pain. You can live with embarrassment. Remorse is an awful companion. And whatever the unwelcome consequences of courage, they are unlikely to be worse than the discovery that you are less than you pretend to be.”

John McCain was no pretender. Instead, he is a living example of someone who – like all of us – would sometimes fall short, but never fail to summon the courage to seek honor and redemption from his toughest critic – himself.

As McCain wrestles with the fear that comes with embracing mortality, I can only imagine that he will personify his definition of courage with a statesman’s grace and the fighter pilot’s moxie:

“Courage is that rare moment of unity between conscience, fear, and action, when something deep within us strikes the flint of love, of honor, of duty, to make the spark that fires our resolve. “It’s the moment — however brief or singular — when we are our complete, best self, when we know with an almost metaphysical certainty that we are right.”

DON’T MISS: The Class Of 2017 Shares Their Biggest Regrets or A Harvard MBA Widow Tells Classmates ‘You Kept Me From Drowning’