15 Stephen Hawking quotes that reveal how a genius thinks

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.