Why did Paul Newman retire from acting in 2007?

Paul Newman retired from acting in 2007 because he felt he could no longer perform at the high standard he demanded of himself. Regarding his decision, he stated, "I’m not able to work anymore as an actor at the level I would want to... so that’s pretty much a closed book to me."1

That same year, Newman was diagnosed with cancer, which ultimately proved fatal in 2008.12