Mens Rea
Mens rea is a Latin phrase meaning "guilty mind." A longstanding principle of criminal law is that a person should only be punished for a crime if two concurrent elements can be proven. The first is the appropriate mental mindset of the accused (the mens rea), and the second is the necessary act or failure to act to accomplish the crime (the actus reus). We say that the mens rea and actus reus need to be concurrent, because traditionally the prosecution needed to prove that the defendant undertook the criminal action at the same time they possessed the wrongful mental mindset.
Most crimes that carry a penalty including incarceration have a mens rea requirement. Offenses handled in the criminal courts of some jurisdictions that are more regulatory in nature, such as traffic offenses, do not have a mens rea requirement and are instead strict liability. In other words, it does not matter that you did not see the light was red when you ran it; all that has to be proven is that you ran the red light. An exception to this in a majority of jurisdictions is statutory rape. Despite statutory rape carrying the possibility of a long prison sentence, most American jurisdictions treat it as a strict liability offense without a mens rea requirement. However, in California, that crime does have a mens rea element. People v. Hernandez, 61 Cal. 2d 529, 536 (1964).
Setting aside the relatively rare strict liability offenses, the mens rea for individual crimes can be divided into two categories: general intent and specific intent.
General intent crimes require that the prosecution prove that the defendant had some general intent to engage in wrongdoing. For example, if a defendant is stealing from a convenience store and, on their way out of the store, they rush past someone, hit them, and knock them over, the defendant could be charged with a battery on that person. Even though the defendant may not have intended to hit that person at all, they were at least reckless as to the possibility they would harm someone while they were in the mindset of doing wrong (in this case, stealing). That recklessness, or what is sometimes called a wanton disregard for the risks of an activity, is often the mens rea that needs to be proven for general intent crimes.
Specific intent crimes require that the prosecution prove that the defendant specifically intended the criminal conduct in question. Specific intent crimes are harder to prove but may also carry stiffer penalties. The sentence someone gets for accidentally killing someone is likely less than the sentence someone receives for hiding in their house and shooting them after a lot of planning. Attempting to commit a crime is generally a specific intent crime. So, paradoxically, it can be more difficult for the prosecution to convict someone of trying to commit a crime than it would be had the person successfully completed the crime.
Many jurisdictions have adopted parts of the Model Penal Code. Under the Model Penal Code, mens rea is termed under following terms, in order of decreasing severity: purposely, knowingly, recklessly, and negligently.
Sometimes the mens rea for a specific offense is not immediately clear. Or, in other cases, the mens rea has been changed by statute. For example, in Wisconsin, First Degree Intentional Homicide merely requires intentional conduct. But in California, the equivalent crime requires malice aforethought, which is both intentional conduct and some level of planning and deliberation.