1. What criminal law does

The function of the criminal law is to set the limits within which the criminal justice system works. That system is a tool of social control. The rules are the rules of social control in a particular society. What is forbidden depends on the corporate morality of that society. Violence, theft and rape are the paradigms. Other conduct is a crime only if that society has decided to treat it as one.

The Wolfenden Report put the purpose in three lines. The law preserves public order and decency. It protects the citizen from what is offensive or injurious. It guards people who are especially vulnerable because they are young, weak in body or mind, inexperienced, or dependent. It is not the function of the law to intervene in private life, or to enforce a pattern of behaviour, further than those purposes require. To that extent the criminal law reflects social morality. The wrong it punishes is the wrong that threatens the values on which the society is founded. An assault or a robbery harms the victim and also the society.

Criminal liability in Sierra Leone is expressed in the maxim actus non facit reum nisi mens sit rea: an act does not make a man guilty of a crime unless his mind is also guilty. There are two elements. The physical element is the actus reus. The mental element is the mens rea. Both must be proved by the prosecution beyond reasonable doubt. A person must do something the law forbids, and must do it with the wrongful state of mind the definition requires.

Lord Goddard called those two elements the cardinal doctrine, in Younghusband v Luftig. Lord Kenyon, in Fowler v Padget, said it is a principle of natural justice that the intent and the act must concur. Lord Hailsham, in Haughton v Smith, put the same point the other way round. It is not the act which is "reus". It is the person, and the mind.

A suggestion that a crime has three elements, the act, the mind, and the absence of a defence, has not been widely accepted. For these chapters, mens rea means the mental element in the definition of the crime. Excuses and justifications are defences. They are not folded into the word mens rea.