One of the cardinal principle of English criminal law is that there must be guilty mind behind that act which is sought to be labeled as criminal. Without the Guilty mind a person is not guilty. They guilty intent and the act must both concur. This is what expressed by the maxim actus non facit reum, nisi mens sit rea i.e. the act by itself does not constitute guilt unless done with guilty intent. A crime punishable under criminal law is not simply an act done an individual, it is an act done with full intent and preparedness. Thus, Mens rea must extend to all the three constituents of an act, viz (i) the physical doing or not doing (ii) the circumstances and (iii) the consequences behind the act.
Only Mens Rea not Punishable:
It should be remembered that mens rea or guilty intention as such is not punishable. Thus, if A has intention to kill B, A cannot be brought to the court of on that ground. Such intention must take some further shape in the form of an act towards the commission of crime. Even if A buys some arms with a view to suing the same in the commission of the crime, still A would not be guilty o any crime towards B, though A may be guilty of keeping arms without proper licence, etc. But if thereafter A is discovered with a loaded gun in the compound belonging to B, then A has done some positive act and he may be guilty under the law. Thus, it is clear that mere presence of mens rea not crime unless such mens rea. However in some cases law awards a punishment even though the actus reus is not consummated e.g. conspiracy, attempt etc.
The word actus connotes a deed i.e. physical result of human conduct. The word reus means forbidden by law. Actus reus, thus, has been defined as such result of human conduct as the law seeks to prevent. When criminal policy of a country regards such a deed as sufficiently harmful, it prohibits it and seek to prevent its occurrence by providing a penalty or punishment for it commission. The deed, so prohibited by law, is known as actus reus. A mere evil intent or design unaccompanied by any overt act which is technically called Actus Reus, in furtherance of such design, is not punishable.