“As a legal principle, mens rea means that causing harm should not be enough to constitute a crime; knowingly causing harm should be. Walking away from the baggage carousel with a suitcase you mistook for your own isn’t theft; it’s theft only if you knew you didn’t own it. Ordinary citizens may assume that this common-sense requirement is already the law of the land. And indeed law students are taught that prosecutors must prove not just that a defendant did something bad, but also that his frame of mind made him culpable when he did it.”