In May 1987, a twenty-three-year-old Canadian named Kenneth Parks rose from his couch, drove fourteen miles across Toronto, and killed his mother-in-law with a tire iron while gravely injuring his father-in-law. He then presented himself at a police station, seemingly unaware that he had severed the tendons in both of his hands during the assault. At trial, expert witnesses testified that Parks had been sleepwalking throughout the entire episode. The jury acquitted him of murder in 1988, and the Supreme Court of Canada upheld that acquittal in 1992. Parks never contested that his body had committed the killing. He contested that he had. The verdict appears indefensible until it is examined against the doctrinal structure it rests upon, which is among the most settled in criminal law. Conviction for a serious offence ordinarily requires the state to establish two distinct elements. The first is the actus reus, the physical commission of the prohibited act. The second is the mens rea, th...