When crimes are defined, they're divided into parts, called elements. And fundamentally, there are two parts, the act element, and the mental element. Actus reus, which is Latin for guilty act, is about someone's external conduct. Mens rea, which is Latin for guilty mind, is about someone's internal state of mind. The law rarely punishes an act without a wrongful state of mind and it never punishes a wrongful state of mind without an act. That is, most crimes include a mens rea term and every crime, literally every single one, includes an actus reus term. The two together, actus reus plus mens rea, are the flint and the steel of criminal law. For example, murder is traditionally defined as killing with malice. The actus reus is killing, the mens rea is malice. Mens rea goes to the very roots of what criminal law is all about. We don't typically punish, under American law, the person who's driving perfectly responsibly and through no fault of her own kills a pedestrian. Maybe the pedestrian just falls and collapses in front of a car. We don’t punish that criminally. We punish the driver who is driving extremely recklessly and runs a pedestrian down, or, worse still, a driver who deliberately runs a pedestrian down. Criminal law is about punishing wrongdoing, and acts that cause harm with an innocent state of mind don’t merit condemnatory blame. Criminal law is highly concerned with wrongful states of mind. American law is filled with different mens rea terms, each with its own definitional nuances. Yet for all its definitional nuance and complexity, American mens rea law comes down, really, to two things: intention and gross negligence. Committing a wrongful act intending the wrongful act, or committing a wrongful act with a level of negligence that shows gross indifference to the rights and welfare of others. Keep those two foundation stones in mind, and you'll be able to navigate the complicated waters of American mens rea law. The key idea with intentional wrongdoing is that you either intended the wrongful conduct, or you intended to bring about the wrongful result. Imagine a foreman at a construction site, it's a hazardous construction site and there are children who play in the area and so it's his job to lock the fence up at night so the children don't get in. One day, he gets a phone call and he's distracted and he forgets to lock up the gate and tragically a child wanders in and falls to his death. Is that foreman civilly negligent for the child's death? Should he or his company be forced to pay damages to the child's family? Undoubtedly, yes. That's an easy case in tort law. Would that foreman count as criminally negligent under traditional definitions of criminal negligence? The answer is not a chance. That doesn't come even close to the level of negligence required to be guilty of negligent homicide. That is to say manslaughter. Now imagine a different scenario. Imagine that site is hazardous and the foreman and the company know that children plays in the area, but they never attempt to take precautions. They never set up a fence. They just take their chances. Well that shows such indifference to the lives of the children playing in the area, that it probably does qualify as criminal negligence and as a manslaughter if a child should die. And now imagine a third scenario, imagine that the foreman never puts up a fence because he wants one of the children to die. He's trying to bring about the death of a child, well that's just murder. That's the wrongful intent required. In all three cases, a child wanders into the construction site and dies. The external situation is precisely the same, what changes is the foreman’s state of mind. And that makes all the difference between a prosecution for murder, a prosecution for manslaughter and a civil suit for tortious wrongful death. Summing up, here’s the takeaway: because criminal law is focused on moral condemnation for serious wrongdoing, it matters what’s inside people’s minds – the psychology of offenders matters. That is why the vast majority of crimes have a mental element – a mens rea term. Mens rea is the greatest tool criminal law has for tracking moral culpability. And while the law of mens rea is complicated, it generally comes down to two things: bad intentions and extreme negligence.