What is a PhD thesis?

King of all nerds, James Gosling, on trying to come up with his PhD thesis topic:

I was falling into a common grad student trap of feeling that I needed to do something grandiose and solve all of the worlds problems. He was into “keep it simple”. So I did, and I came up with a pretty straightforward thesis proposal. The odd thing was that when I finally finished my thesis, I realized that I had only delt with one sentence out of the simplified proposal.