What is the books definition of evil?

The book defines evil as a reality within our world that is apparent in the temptation that surrounds us.

More than anything evil is taken as the way in which we give in to temptation. The clowns, which seem to be the physical manifestation of evil, do have power only when humans believe in their evil. There is a sense of evil in the novel throughout the novel, but in the end humans create their own evil.