I am flying to see Archbishop Tutu in South Africa and then on to Professor Hawking’s memorial, where he will be laid to rest at Westminster Abbey next to Charles Darwin and Sir Isaac Newton. I can’t sleep on this second of two back-to-back redeye flights, and I keep thinking of these two men. I am not thinking of their greatness or their accomplishments or what they have given to the world, but of their weaknesses and vulnerabilities, of Hawking’s eyes and Tutu’s arm.

I was not prepared for the beauty of Professor Hawking’s eyes. When we met at Cambridge last year, his voice-synthesizing software was malfunctioning and his typically labored, computer speech, which he controlled with an eloquent cheek muscle, was not available to him. He was reduced to communicating yes or no with his eyebrow. His UK-based agent, Robert Kirby, and I had suggested gathering his pieces and statements from his personal archive so we could offer a small volume, Brief Answers to the Big Questions. Fortunately, after a rather longwinded explanation on my part about the project we wanted to help him create, he raised his eyebrow, and his personal assistant, Anthea Bain, translated that he had said yes. He wanted the project to move forward.

Because of his challenges in communicating, all of his answers are necessarily brief, but he has not let these limitations prevent him from telling the world about the big cosmological questions: Where do we come from? Can we travel through black holes. Might aliens exist? Is there a God? And increasingly in his last years, he has tried to answer more urgent questions: Can we colonize other planets? How might educate our children and protect our most vulnerable? Will we survive? But as I was describing the importance of his work and sharing his latest messages with the world, I kept finding myself distracted, absorbed, by his blue piercing eyes, which were mesmerizing.

Struck by Motor Neurone Disease at twenty, he was at 76 the longest-lived survivor of the dreaded disease, which had also killed my uncle. Called ALS or Lou Gehrig’s disease in America, it had robbed him of all the capabilities and physical strengths that we depend on for our independence and self-identity. I had been warned that he could be very strong-willed, perhaps as a result of his almost total dependence on others. Falling into his eyes was like being drawn irresistibly into the gravitational force of black holes. And what I saw there was the radiance and beauty of consciousness itself. Even as all of the other charms and graces had been stripped from him, what remained was the mind’s quest to know, to discover, and to survive. And, oh God, was that that drive magnificent, more beautiful than the shape of any muscular body.  As a young nerd, he wasn’t the most handsome man at Cambridge, but as an elderly scientist, his mind, his will—might one dare to say something as unscientific as his soul—was drop-dead gorgeous.

I remember the first time I went for a drive with Archbishop Tutu, and he reached over with his left hand to turn the ignition on the right side. He explained that his right hand had been made infirm by polio as a boy. I was struck by how many obstacles and weaknesses he had confronted in his life, from polio and tuberculosis to racism and apartheid, but these challenges did not stop him, they burnished him, they forged him, they sculpted his soul, a term he is much more comfortable using than Professor Hawking. Archbishop Tutu has often reminded me that our weaknesses and our limitations are a reminder of our need for one another. They are not to be shunned but embraced. To be human is to be vulnerable, frail, wounded.

I’ve been very fortunate and have not faced anything like the challenges endured by these two men. As a child, I dealt with dyslexia and thought my brain was somehow broken, since this handicap was a source of struggle and limitation in school. It was a source of shame, as my brain did not work quite like other people’s. And it was therefore a rather strange decision to go into writing and publishing. But we are often drawn to our challenges like a moth to a flame, returning, even unconsciously, to try to confront them, to transcend them, to learn from them. Archbishop Tutu’s own vulnerabilities had given him a deep compassion for those of others.

When I was interviewing Archbishop Tutu and the Dalai Lama in Dharamsala for The Book of Joy,we threw a birthday party for His Holiness’s 80th. We arrived for the celebration at one of the schools that the Dalai Lama had established to educate refugee children because they are not allowed to get a Tibetan education in Tibet. Parents would send their children, often as young as five, over the mountain passes with guides or relatives to the boarding schools, not knowing if they would see them again for thirteen years, if ever. The children told us harrowing, tear-filled stories of leaving their families and hiding from the Chinese police. As we entered the room, one of the boys was bowed forward, his hands pressed together at his heart in the traditional pose of respect and greeting. His sweet face and soft brown eyes looked up at the Dalai Lama in a mixture of awe and nervousness. The Dalai Lama stopped in front of the child, even as the whole parade of people was moving forward, and touched a scar on the boy’s cheek with his thumb, gently, caringly. Then he pointed to a scar on the top of his shaved head and chuckled. I don’t speak Tibetan so don’t know the words he spoke as he laughed, but in the international language of gestures, it sounded a lot like, “I have one too.” It was an example of how we can show our scars, our weaknesses, our vulnerabilities to one another and remind each other that they are not a source of shame but of pride. They are after all what make our lives textured, rich, even triumphant.

Professor Hawking’s disability had not stopped him but had forced him to pursue with Olympic-level talent the one arena where he could perform—the mind. Our final frontier. How we deal with our mind determines the person we become far more than the body that withers, weakens, and falls away. This universe, he might tell us, is a laboratory of gravity, where we fall down, learn, and get up, stronger. The world will miss you, Professor Hawking, for so many reasons including your extraordinary example of the human spirit’s ability to confront and ultimately to journey beyond our human frailty and limitations.