Imagine that, thinking you were alone, you were carelessly swinging a baseball bat around, hitting imaginary home runs. Unknown to you, however, a neighbour had walked up to talk to you and your bat struck her square on the head, sending her to the hospital emergency ward. Would you be guilty of a crime of assault?

If you answered no to the preceding question, you were right. Why? Because even though you struck your neighbor, you had no intention of hurting her. In our criminal justice system, a person is not normally held responsible for a crime unless two things are present simultaneously:

• the actual act (or, in some cases, a failure to act), voluntarily carried out, and
• the intention of committing the act, understanding its likely consequences.
  
  
  

In criminal law, Latin terms are used for these two essential elements of a crime. Actus reus (literally, the act of the thing ) is the term used to mean "a guilty act", and mens rea (literally, the mind of the thing ) is the term used to mean "a guilty mind" - in other words, usually an intention to commit a misdeed. Both elements must be present - and at the same time. You can intend to murder someone but fail to do so, or you can accidentally kill someone with no intention whatsoever. In neither case are you guilty of the crime of murder.