Stephen Hawking was 21 years old when he was diagnosed with motor neuron disease, a rare form of ALS.
Doctors told him he would only live for a few years.
More than 50 years later, he's now 73 and one of the foremost physicists alive — a professor at the University of Cambridge, an investigator of black holes, and the author of the bestselling book "A Brief History of Time."
Here are 15 quotes showing Hawking's approach to science and to life in general.
"My advice to other disabled people would be, concentrate on things your disability doesn’t prevent you doing well, and don't regret the things it interferes with. Don’t be disabled in spirit, as well as physically."
"My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all."
On free will
"I have noticed that even people who claim everything is predetermined and that we can do nothing to change it, look before they cross the road."
"Life would be tragic if it weren't funny."
On black holes
"If you jump into a black hole, your mass energy will be returned to our universe but in a mangled form which contains the information about what you were like but in a state where it can not be easily recognized. It is like burning an encyclopedia. Information is not lost, if one keeps the smoke and the ashes. But it is difficult to read."
On the value of string theory
"When we understand string theory, we will know how the universe began. It won't have much effect on how we live, but it is important to understand where we come from and what we can expect to find as we explore."
On his IQ
"I have no idea. People who boast about their IQ are losers."
On what he thinks about all day
"Women. They are a complete mystery."
On why he writes for a popular audience
"I put a lot of effort into writing 'A Briefer History' at a time when I was critically ill with pneumonia because I think that it's important for scientists to explain their work, particularly in cosmology. This now answers many questions once asked of religion."
On the Eureka moment of a new discovery
"I wouldn't compare it to sex, but it lasts longer."
His advice to his three children
"One, remember to look up at the stars and not down at your feet. Two, never give up work. Work gives you meaning and purpose, and life is empty without it. Three, if you are lucky enough to find love, remember it is there and don't throw it away."
"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans. We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."
On his health
"When I was first diagnosed with ALS, I was given two years to live. Now 45 years later, I am doing pretty well."
"God may exist, but science can explain the universe without a need for a creator."
On hitting roadblocks
"It is no good getting furious if you get stuck. What I do is keep thinking about the problem but work on something else. Sometimes it is years before I see the way forward. In the case of information loss and black holes, it was 29 years."
Read next on Business Insider: Here's what Eddie Redmayne, who played Stephen Hawking in 'The Theory of Everything,' said about his death