Many criminal laws require a person to “knowingly” engage in illegal activities. The part of the crime that must be committed knowingly depends on the crime. For example, a drug trafficking law could require the person to “knowingly” import an illegal drug into the United States. If the defendant had received a gift to deliver to someone in the United States and he honestly did not know that the gift contained an illegal drug, then mens rea or necessary mental state was not established and no crime had been committed. While the notion of criminal intent is used around the world, the term mens rea is specific to Anglo-American criminal law. The mens rea concept was born in England at the beginning of the 17th century. It is based on the idea that it is morally reprehensible to punish a person for harm innocently and unknowingly done to society. To be guilty, the criminal must have committed his act in a guilty state of mind. Thus, while an indictable act (actus reus) is an essential element in determining whether a crime has actually been committed, a person can be found not guilty if mens rea is missing.
(An exception to this rule is found in cases of strict liability.) The concept of mens rea developed in England in the second half of the common law era (around the year 1600), when judges began to rule that an act alone can only establish criminal responsibility if accompanied by a guilty state of mind. The degree of mens rea required for a particular common law crime varied. Murder, for example, required a malicious state of mind, while larceny required a criminal state of mind. The court will have no difficulty establishing mens rea when there is actual evidence – for example, if the accused has made an admissible confession. This would be sufficient for a subjective test. But a significant proportion of those accused of a crime do not make such a confession. Therefore, a certain degree of objectivity must be used as a basis for assigning the necessary components. It is always reasonable to assume that people with ordinary intelligence are aware of their physical environment and the ordinary laws of cause and effect (see causation). Thus, if a person plans what to do and what not to do, they will understand the range of likely outcomes of a particular behaviour on a sliding scale from “inevitable” to “likely” to “possible” to “unlikely”. The closer an outcome is to the “inevitable” end of the scale, the more likely it is that the defendant foresaw and desired it, and thus the more certain it is to assume intent. If there is clear subjective evidence that the defendant did not have foresight, but a reasonable person would have done so, the hybrid test may establish criminal negligence. With respect to the burden of proof, the requirement that a jury have a high level of certainty prior to conviction is defined as “beyond a reasonable doubt” in the United States and “safe” in the United Kingdom.
It is this reasoning that justifies the defence of childhood and mental disability under the M`Naghten Rules, an alternative common law rule (e.g., Durham Rule) and one of the various statutes that define mental illness as an excuse. Moreover, if there is an irrebuttable presumption of doli incapax, i.e. that the accused did not have a sufficient understanding of the nature and quality of his acts, the mens rea required is lacking, regardless of the probability that it would have been otherwise. Therefore, for these purposes, if the relevant statutes are silent and it is for the common law to establish liability, the reasonable person must be endowed with the same mental and physical qualities as the defendant, and the test must be whether a defendant with those specific qualities would have had the necessary foresight and desire. Mens rea is the Latin term and means “guilty spirit”. In criminal law, mens rea means the state of mind required to commit a crime. Mens rea is usually divided into four categories, as different crimes require different levels: the motive cannot be defense. For example, if a person breaks into a laboratory where drugs are tested on animals, the question of guilt is determined by the presence of actus reus, i.e. involuntary entry and property damage, and a mens rea, i.e. the intention to enter and cause the damage.
The fact that the person may have had a clearly articulated political motive to protest against such tests has no bearing on responsibility. If the reason is relevant, it may be discussed in the trial portion of the trial when the court determines what sentence, if any, is appropriate. Mens Rea, in Anglo-American law, criminal intent or evil spirit. In general, the definition of a crime includes not only an act or omission and its consequences, but also the mental state of the perpetrator accompanying it. All criminal systems require an element of criminal intent for most crimes. However, only Anglo-American systems use the term mens rea. Countries such as France and Japan simply stipulate that there must be criminal intent, unless a specific law provides otherwise. “The full definition of any crime contains, explicitly or implicitly, a statement about a state of mind. Thus, if it is established that the mental element of the conduct allegedly constituting a criminal offence is lacking in a particular case, the offence so defined is not committed; or, when a crime is fully defined, nothing constitutes that crime that does not conform to that definition. In English law, section 8 of the Criminal Justice Act 1967 provides a legal framework within which mens rea is assessed.
It states: The first level, intent, requires that the defendant intends to achieve the concrete result. First-degree murder and most robbery crimes often require this level of mens rea, although most crimes are not. The vast majority of law enforcement in the United States is conducted by the various states in accordance with the laws of that state. Historically, states (with the partial exception of Louisiana civil law) applied common law rules of mens rea, similar to those that existed in England, but over time the American understanding of common law mens rea differed from that of English law and from each other. From the late 1950s to the early 1960s, the common law of men was widely recognized as a slippery, vague and confusing mess.  This is one of the many factors that led to the development of the Model Penal Code. Thus, the actus reus and mens rea of homicide in a modern penal code can be considered as follows: the levels of mens rea and the distinction between them vary from jurisdiction to jurisdiction. Although the common law originated in England, the common law of each jurisdiction varies in terms of guilt, as precedents and statutes vary.