What constitutes a criminal offence?

A criminal offence is made up of two parts: The Actus Reus and the Mens Rea. The Actus Reus (AR) is the physical part of the crime, the act as such. For example, the application of unlawful force is the Actus Reus of a Battery within common assault, meaning that if I unlawful touch someone, for example a punch within a fight, this constitutes the AR element of an offence. The Mens Rea (MR) on the other hand is the mental element of the crime, what you were thinking and intended while carrying out the act. The literal translation of Mens Rea is Guilty Mind. To continue with the common assault example, the MR required for Battery is 'recklessness or intention' as to the application of the unlawful force. Within the fight example if the punch was deliberate, or was at least reckless, then this would make up the offence of Battery.