Can You Kill Grandpa?

The grandfather paradox is the idea that you can't go back in time and kill a person. The problem has been solved by David Lewis, who says we don't have to choose our pasts. But this contradicts some of origin's comments about not being able to make choices based on god's beliefs. This article was originally published as an extract from lewis' book "A Man Walks On To A precipice" (Simon & Schuster, 2010)