The former is primarily concerned with the 'physical' aspect of criminal responsibility whereas the latter is concerned with the 'mental' aspect.
In understanding actus reus one must establish from the very beginning that it constitutes something much wider than just a criminal act. In some instances failure to act or omission constitute actus reus of an offence. It is usual that just a physical act is required to satisfy actus reus requirements. However, in certain circumstances must prevail and certain consequences must follow from the action/s of the accused.
In relation to statutory offences, such statutory provisions and decided cases that have interpreted them will indicate what constitutes the actus reus of a particular offence but may not necessarily indicate that whether or not liability can arise by means of an omission.
Accordingly a useful working definition of actus reus is that it comprises all the elements of the definition of the offence except those that relate to the mental element (mens rea) which is required on the part of the accused. (Allen, 1991, 18).
In understanding the term mens rea it is not easy to decide on the evidentiary criteria for something as hard to define as mental state of a given offender. They are physical control at the time the act took place. Some terms of mens rea known are volition meaning knowledge of the relevant circumstances and foresight of the consequences.
In most crimes if both elements actus rea and mens reus are not present then there is no crime committed. However, in some crimes it does not embrace the above rule and this are offences of strict liability and inchoate cases.
The extent to which, if any, strict liability and inchoate offences differ from the normal requirements of a crime
. In any legal system most crimes must be prescribed to a physical element or a wrongful act commonly known as actus reus; and the mental element or guilty mind known as the mens rea. As reiterated above if both elements are not present then there is simply no crime.
In contrast, strict liability offence is one in which there is no requirement of mens rea in relation to one or more elements of the actus reus. In other words, it focus on the harm done.
In the UK jurisdiction, most strict liability offences are statutory in origin and are often described as regulatory in nature. Therefore there are strict liability offences within the realm of law relating to issues of pollution Alphacell v Woodward  AC 824.
In the above case, the defendants were convicted under section 2(1) of the Rivers (Prevention of Pollution) Act 1951 for causing polluted water to enter a river. It was held that the fact they had no knowledge that the pollution was taking place and that they mistakenly thought that their filtering system was operating effectively did not constitute a defence.
The existence of such offences and the fact that persons can be criminally liable without the prosecution being under any obligation to address the issues of mens rea in relation to one or more elements of the actus reus of an offence would seem to be in direct conflict with everything that we have discussed initially in this paper.