Paper delivered at the Moral Evil in Practical Ethics Conference, Oxford 2012. I argue that there is a secular problem of evil analogous to the well-known theological problem of evil; give a definition of evil; consider two widely held inadequate explanations of it; and propose a better explanation. The critical aim of the argument is to argue that the prevalence of evil is a reason for rejecting the optimistic faith shared by numerous past and present thinkers. Its constructive aim is to explain evil as a result of ambivalence that is inherent in the human condition.