The relationship between actus reus and mens rea goes to the heart of understanding and judging any crime. These two separate issues are often referred to as the two elements of any crime, though the amount of influence they exert over a legal trial may vary depending on the circumstances. Actus reus refers to the physical action of a crime, while mens rea refers to the intent and understanding of the criminal at the time of the crime.
How actus reus and mens rea interact help determine the culpability of a criminal. While the actus reus determines whether an act is unlawful, the mens rea tries to define the level of moral responsibility. Under some legal systems, for instance, a homicide charge only applies to a murder in which the perpetrator knowingly and willingly committed the killing. While the perpetrator killing the victim is the actus reus of the crime, it is the mens rea that may determine whether the killing is defined as a homicide, manslaughter, or accidental death.
Unfortunately, using actus reus and mens rea to determine a criminal penalty is rarely cut and dry. As philosophers have long noted, it is impossible to tell or even fully understand what goes on in the mind of another person. Juries and judges are often reduced to inferring the mens rea through the circumstances of the crime as well as the arguments and testimony of each side. This can make the assigning of motives and intent a murky business, regardless of the actus reus.
In determining mens rea for a crime, judges and juries are often given the opportunity to examine the motive based on objective and subjective standards. The objective standard asks if a reasonable person would have known that the act is illegal, and would have made the same choices in the given circumstances. The subjective standards relies on the court's understanding of the actus rea; essentially asking if the evidence allows the inference of intent beyond a reasonable doubt. Examining the issue against these concepts can help a judge or jury make a fair inference as to the mens rea of the defendant. In such cases, the actus reus and mens rea are indelibly connected to one another.
In many crimes, the actus reus and mens rea do not carry equal weight. Some criminal acts, known as strict liability offenses, do not take mens rea into account at all. It is also usually not permitted to argue that a criminal charge should be dismissed or reduced because the defendant did not know the law; though in cases where laws are notably complex and considered beyond the comprehension of a reasonable person, such as tax laws, this type of mens rea argument may be granted some weight.