Criminal liability is based upon a combination of actions (actus reus) and thoughts (mens rea).
This is expressed by the maxim actus non facit reum nisi mens sit rea which means that an act alone will not give rise to criminal liability unless it was done with a guilty state of mind.
If actus reus and mens rea are established and there is no valid defence, the defendant is guilty. The onus is on the prosecution (burden of proof) to establish the elements of the offence beyond reasonable doubt (standard of proof).
Actus reus and mens rea
These are the building blocks of criminal liability. In simple terms, actus reus (AR) is the guilty act and mens rea (MR) is the guilty mind, both of which are required for criminal liability.
The precise nature of the actus reus and mens rea are determined by the particular offence. For example, the actus reus of criminal damage is the damage/destruction of property belonging to another whilst the mens rea of murder is intention to kill or cause GBH (grievous bodily harm). The definition of the offence, in statute or common law, will contain the elements of the offence