"I love to kill people. I love watching them die." So said Richard Ramirez, the notorious Los Angeles "Night Stalker." Can we say that he, and others like him, are pure evil? Or, is that just a matter of personal opinion? This book is about morality. It's about proving good and evil are real; that they're not just an opinion; that they are based on a measurable standard; that the measurable standard comes not from religion, not from philosophy, but from science. What is that scientific standard? Symmetry. It's how things work in physics, in aesthetics, and even in ethics. The same symmetry that governs the interaction among subatomic particles, that forms the wings of a butterfly, that determines the genius of a snowflake, also has fashioned you and me as moral beings, and has given us the choice between what's evil and what's good. That's what I will explain.