Criminal justice is a complex system that is based on a set of core principles. These principles dictate what constitutes a crime and how it should be punished. The most important of these principles is the rule against retroactivity, which prohibits the imposition of laws ex post facto (i.e., laws that are applied retroactively). This rule restricts the authority of judges to declare new crimes, although they can expand the scope of old crimes through interpretation.
Another key principle is that a person cannot be convicted of a crime without having intended to commit the act in question. With few exceptions, the person does not need to know that the act itself is a crime, since ignorance of the law is no excuse for criminal conduct. Therefore, if a person believes that an act is perfectly legal and performs it intentionally, the legal requirement of criminal intent is met. Insanity is another condition that can exempt people from criminal liability.
It is defined as a mental illness or defect that makes a person not know what they are doing or that they are not aware that what they are doing is wrong. A legal declaration of insanity results in the acquittal of criminal charges (“innocent by insanity”), because the person lacks the required intention. Automatism is another rare condition that totally exempts people from criminal liability. It occurs when the conscious mind does not control body movements, such as during somnambulism, which makes the person unaccountable even for serious consequences. The principle of criminal intent is subject to many other exceptions and requirements.
In some cases, such as crimes of strict liability, it is completely abandoned or only a limited scope is allowed. For example, employers can be held responsible if employees are injured on the job, regardless of how carefully they have followed safety precautions, and manufacturers can be held responsible for injuries caused by a defective product, even if they showed no fault or negligence in the manufacturing process. In some cases, such as murder, evidence of a high degree of provocation (in English law, sufficient to cause a reasonable person to act in the same way as the accused) could result in a murder sentence, even if the murder was intentional. The practice of not absolving people with mental disabilities but mitigating their punishments is found in many common law countries. The fact that a person has been drinking or using drugs before committing a crime is not in and of itself a defense, except possibly in the case of crimes that require specific intent. Provocation is not generally a defense either, except in cases of murder. Criminal liability applies not only to those who commit criminal acts but also to those who knowingly encourage or abet an author by knowingly encouraging or helping him to commit such an act (e.g., providing assistance or advice).
Generally, everyone is held equally responsible and subject to the same penalty. However, in many cases, an accomplice before the fact is considered more guilty than an accomplice after the fact. In some jurisdictions (e.g., England), those who actually commit the criminal act (e.g., robbery) can be held accountable for any death that occurs during its commission (e.g., first-degree murder). This is similar to strict liability. In conclusion, there are several core principles of criminal justice that are widely found in all criminal justice systems. These include the rule against retroactivity, which prohibits ex post facto laws; the requirement of criminal intent; insanity and automatism as conditions that exempt people from criminal liability; and strict liability for certain crimes.
