Legally speaking, a crime is 'an action or omission that constitutes an offense that may be prosecuted by the state and is punishable by law'. A crime is an act that gives rise to criminal liability, and the offender can be prosecuted in accordance with the criminal laws of the state. So, a crime can be an action of omission. Theft, murder, robbery, or kidnapping, are some of the actions that are classified as crimes. An omission is considered as a crime if the offender fails to act when he has a legal or moral duty to do so. Failure to pay taxes is an example of omission, that amounts to a crime. Failure to report the death of a child or the location of a corpse are also omissions that give rise to criminal liability. If parents fail to take care of their kids, such an omission may also be considered a crime. Crime is a generic term that encompasses a host of acts and omissions that are punishable by the criminal laws of the state. A crime can be a murder, kidnapping, robbery, or failure to pay taxes. In general, crimes have some basic elements that have to be proved to establish criminal culpability. Apart from that, each type of crime has a set of specific elements that have to be proved so as to obtain conviction. By now, it is clear that each and every crime has an action or omission that is prohibited by law. So, a prohibited conduct is a basic element of a crime. Such a conduct or action is termed as 'actus reus'. According to common law, a person cannot be made criminally liable if he/she lacked criminal intent while committing an act. 'Mens rea' is the term used to denote such criminal intent. Though there are exceptions to this rule, most crimes require the element of mens rea to be proved. So, actus reus and mens rea constitute the basic elements of a crime. Actus reus is defined as 'the wrongful act or omission that comprises the physical components of a crime'. Though actus reus denotes the physical act, in fact, it includes the conduct of the offender, the circumstances of the crime, and the consequences of the act. Mens rea denotes the mental state of the offender while committing the crime. The term means 'a guilty mind; a guilty or wrongful purpose; a criminal intent; guilty knowledge and wilfulness, etc'. While actus reus denotes the physical element of a crime, mens rea refers to the mental element. While the physical act' or the actus reus, may vary from one crime to another, the mental element, or mens rea, may exist in different degrees. Both elements need to be proved so as to establish the criminal liability of a suspect. Proving the mental element is not required to establish liability in civil litigation. However, in some cases (like certain torts), if it is proved that the act/omission was done 'intentionally', the defendant's liability to pay damages may increase. Example 1: A and B are close friends. A was practicing in a secure firing range, when B came in accidentally. B got shot in the head by the bullet that came out of A's gun. B died. Example 2: A and B have been married for the last five years. One day, A came to know that B has some extramarital affair. A shot B while the latter was in deep sleep. B died on the spot. If you go through these examples, it is really easy to make out the actus reus - shooting the victim using a gun. In both cases, the victims were killed. What is the difference between these two instances? In the first example, 'A' lacks the mental element or mens rea, that is required to make him liable for the criminal act. In the first example, the act was done unintentionally; whereas, the act of 'A' was intentional in the second instance. The intentional action amounts to mens rea, which makes the offender liable. Assault is an offense that is defined as 'An intentional act by one person that creates an apprehension in another of an imminent harmful or offensive contact'. In this case, the act of the offender that creates an apprehension of imminent fear in the victim (the physical act of threatening to cause harm) amounts to actus reus. The intention to create such an apprehension in the victim amounts to mens rea. Degrees of Mens Rea: The different degrees of mens rea and their definitions may vary from one jurisdiction to another. In general, the different levels are described as negligently, recklessly, knowingly, and purposefully. (a) Purposefully: A person acts intentionally, with a purpose, thereby expecting certain results. (b) Knowingly: A person acts with the knowledge that such an act will certainly cause a specific result. (c) Recklessly: A person acts, ignoring the substantial risk because of which the act is prohibited, and will lead to a prohibited result. (d) A person who should have been aware of the prohibitory nature of the act and its results acts without such awareness. In short, mens rea plays a major role in determining the criminal liability of a person. A person cannot be deemed guilty if he lacked the criminal intent while committing an act. This common law doctrine is expressed through the maxim 'actus non facit reum nisi mens sit rea', which means that an act does not make one guilty unless his mind is guilty. The prosecution must prove that the defendant had criminal intent (mens rea) at the time of committing the crime. The rule is that the suspect must have mens rea either before or while doing the crime. So, actus reus and mens rea must be concurrent (simultaneous) or motivational (actus reus motivated by mens rea). In some cases, it may not be that easy to apply the rule of concurrence. The timing of formation of mens rea may vary in some cases, and this may make it difficult to apply this rule. The principle of concurrence or coincidence is applicable for an act which comprises a single event, or an act that comprises a series of events. In case of a series of events that are continuous, the mens rea must be formed either before or during the events. For example, 'A' was driving his car during the middle of night, when he hit 'B' accidentally. 'A' leaves the place, even though he finds 'B' lying unconscious on the road. After some time, another car hits 'B', who dies on the spot. In fact, the driver of the second car did not see 'B' lying on the road. In this case, 'B' would not have died if 'A' had not left him on the road. It is reckless omission (to move 'B' from the road) on the part of 'A' that lead to the death of 'B'. This amounts to mens rea' that makes 'A' liable. So, the series of events will constitute the actus reus, and mens rea will be deemed to have formed during the continuing series of events. In short, both are the most important and basic elements when it comes to establishing criminal liability. The difference between them is that' one represents the physical element (actus reus)' and the other represents the mental element (mens rea) of a crime. In some cases, a person may become liable for his acts or omissions, irrespective of the mental element. In other words, proving mens rea is not necessary to make a person liable in certain cases. A person will be deemed liable and will be convicted even if he is not aware that his action is prohibited in nature. Usually, this rule of strict liability is applied in minor offenses like traffic violations.