One of the foundations of criminal law is that the actus reus must consist of a voluntary act for a criminal defendant to be culpable. It puzzled me at first too. Read on! You are familiar with many types of actions the law considers involuntary. For example, actions performed while asleep, somnambulistic actions, are involuntary. So if you drove a spike through your husband's head while you were asleep you can't be guilty of murder because there is no voluntary action, no actus reus!

Reflexes are the most obvious type of non-action (i.e. an action that can't qualify as an actus reus). Some consider shock a natural reflex. So if you killed someone while you were in shock, then you can't have acted voluntarily and can't be criminally culpable. If Fred killed Barney with one of his massive foot-swallowing, knee-jerk reflexes, then Fred would not be criminally liable. He performed no culpable act, no actus reus, on which to base liability.