My PhD dissertation and first book were about the proslavery argument. You might say I wanted to understand inhumanity—how men and women throughout history have persuaded themselves to defend ideas, practices, societies, governments that we of a different era see as indefensible. I wanted to know how humans can become blind to evil. Perhaps if we could understand their processes of denial and rationalization we might gain insight into our own failures of vision, the shortcomings of our own time.History, in other words, can expand our awareness of ourselves. It releases us from the confines of our own individual lives; it offers us other ways of seeing that cast our assumptions into relief. It reminds us of choices people have made—or not made—and thus illuminates realms of possibility. It shows us that things have been otherwise and reminds us they can be different once again. By documenting contingency and agency, history undermines any acceptance of crippling inevitability. And contingency means opportunity. It means that we can change things and that what we do matters. To my mind this may be history’s most important lesson.