Apple CEO Tim Cook has commemorated the life of theoretical physicist Stephen Hawking, who died early Wednesday at the age of 76. "We will always be inspired by his life and ideas," he said on Twitter.

Stephen Hawking via REX/Shutterstock

“The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.” -Stephen Hawking. We will always be inspired by his life and ideas. RIP. — Tim Cook (@tim_cook) March 14, 2018

Hawking was a renowned scientist, cosmologist, astronomer, and mathematician. He authored several books, including his best-selling 1988 classic A Brief History of Time , which has sold more than 10 million copies.

Hawking was diagnosed with the degenerative disease amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, at the age of 21. He was given just a few years to live, but battled the illness for more than five decades.