Sentence:  The majority of serious criminal offences require, in addition to the "actus reus", a specific state of mind on the part of the accused, usually referred to as the <WSD>"mens rea</WSD>". Many less serious crimes require no <WSD>"mens rea</WSD>", but simply proof that the defendant caused the prohibited harm. 
Meaning of the word marked with <WSD> tags:  A guilty mind, the conscious knowing of a perpetrator while committing an act that the act is illicit.