Undoubtedly the greatest physicist of our times, Stephen Hawking died at the age of 76. His work on the origin of the universe and black holes was groundbreaking and A Brief History Of Time remains on bestseller lists two decades after it came out (albeit famously unread). But the scientist with the sharp mind and unique wit was full of surprises – starting in 2007 he co-wrote four children’s books along with his daughter Lucy Hawking. In his own words, here is the best of Hawking from the invaluable body of work he leaves behind.

On being diagnosed with an incurable disease

Not knowing what was going to happen to me or how rapidly the disease would progress, I was at a loose end. The doctors told me to go back to Cambridge and carry on with the research I had just started in general relativity and cosmology. But I was not making progress because I didn’t have much mathematical background—and anyway, it was hard to focus when I might not live long enough to finish my PhD. I felt somewhat of a tragic character.

I took to listening to Wagner, but reports in magazine articles that I also drank heavily at that time are an exaggeration. Once one article said it, other articles copied it because it made a good story, and eventually everyone believed that anything that had appeared in print so many times must be true.

My dreams at that time, however, were rather disturbed. Before my condition was diagnosed, I had been very bored with life. There had not seemed to be anything worth doing. But shortly after I came out of the hospital, I dreamed that I was going to be executed. I suddenly realized that there were a lot of worthwhile things I could do if I was reprieved. Another dream I had several times was that I would sacrifice my life to save others. After all, if I was going to die anyway, I might as well do some good.

From My Brief History: A Memoir, published in 2013.

On his discovery of how black holes work

From 1970 to 1974 I worked mainly on black holes. But in 1974, I made perhaps my most surprising discovery: black holes are not completely black! When one takes the small-scale behaviour of matter into account, particles and radiation can leak out of a black hole. A black hole emits radiation as if it were a hot body.

Since 1974, I have been working on combining general relativity and quantum mechanics into a consistent theory. One result of that has been a proposal I made in 1983 with Jim Harde of the University of California at Santa Barbara: that both time and space are finite in extent but they don’t have any boundary or edge. They would be like the surface of the earth, but with two more dimensions. The earth’s surface is finite in area, but it doesn’t have any boundary. In all my travels, I have not managed to fall off the edge of the world. If this proposal is correct, there would be no singularities, and the laws of science would hold everywhere, including at the beginning of the universe. The way the universe began would be determined by the laws of science. I would have succeeded in my ambition to discover how the universe began. But I still don’t know why it began.

From Black Holes and Baby Universes and Other Essays, published in 1993.

On why he wrote A Brief History of Time

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the centre of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on?” “You’re very clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”

Most people would find the picture of our universe as an infinite tower of tortoises rather ridiculous, but why do we think we know better? What do we know about the universe, and how do we know it? Where did the universe come from, and where is it going? Did the universe have a beginning, and if so, what happened before then? What is the nature of time? Will it ever come to an end? Can we go back in time? Recent breakthroughs in physics, made possible in part by fantastic new technologies, suggest answers to some of these longstanding questions. Someday these answers may seem as obvious to us as the earth orbiting the sun – or perhaps as ridiculous as a tower of tortoises. Only time (whatever that may be) will tell.

From A Brief History of Time, published in 1988.

On giving time a shape

What is time? Is it an ever-rolling stream that bears all our dreams away, as the old hymn says? Or is it a railroad track? Maybe it has loops and branches, so you can keep going forward and yet return to an earlier station on the line.

The nineteenth-century author Charles Lamb wrote: “Nothing puzzles me like time and space. And yet nothing troubles me less than time and space, because I never think of them.” Most of us don’t worry about time and space most of the time, whatever that may be; but we all do wonder sometimes what time is, how it began, and where it is leading us.

Any sound scientific theory, whether of time or of any other concept, should in my opinion be based on the most workable philosophy of science: the positivist approach put forward by Karl Popper and others. According to this way of thinking, a scientific theory is a mathematical model that describes and codifies the observations we make. A good theory will describe a large range of phenomena on the basis of a few simple postulates and will make definite predictions that can be tested. If the predictions agree with the observations, the theory survives that test, though it can never be proved to be correct.

On the other hand, if the observations disagree with the predictions, one has to discard or modify the theory. (At least, that is what is supposed to happen. In practice, people often question the accuracy of the observations and the reliability and moral character of those making the observations.) If one takes the positivist position, as I do, one cannot say what time actually is. All one can do is describe what has been found to be a very good mathematical model for time and say what predictions it makes.

From: The Universe in a Nutshell, published in 2001.

His surprising writings for children

“Let me show you something fun,” Eric shouted, producing a plastic ruler from his pocket. He brandished it in front of George’s nose. “Do you know what this is?” he asked at top volume.

“A ruler?” said George. The answer seemed a bit too obvious.

“That’s right,” cried Eric, who was now rubbing the ruler against his hair. “Watch!” He held the ruler near the thin stream of water running from the faucet. As he did so, the stream of water bent in the air and flowed at an angle rather than straight down. Eric took the ruler away from the water and it ran down normally again. He gave the ruler to George, who rubbed it in his hair and put it close to the stream of water. The same thing happened.

“Is that magic?” yelled George with sudden excitement, completely distracted from Annie’s rudeness. “Are you a wizard?”

“Nope,” said Eric, putting the ruler back in his pocket as the water ran down in a long straight line once more. He turned off the faucet and switched off the radio. It was quiet now in the kitchen, and Annie could no longer be heard in the distance.

“That’s science, George,” said Eric, his whole face shining. “Science. The ruler steals electric charges from your hair when you rub the ruler through it. We can’t see the electric charges, but the stream of water can feel them.”

“Gosh, that’s amazing,” breathed George.

“It is,” agreed Eric. “Science is a wonderful and fascinating subject that helps us understand the world around us and all its marvels.”

From George’s Secret To The Universe, co-written with Lucy Hawking, published in 2007.