Before a court finds a defendant guilty of a criminal offense, the prosecution must present evidence that, even when opposed by any evidence the defense may choose, is credible and sufficient to prove beyond a reasonable doubt that the defendant committed each element of the particular crime charged.
However, for some legislatively enacted crimes, the most notable example being statutory rape, a defendant need not have had any degree of belief or willful disregard as to the existence of certain factual circumstances (such as the age of the accuser) that rendered his conduct criminal; such crimes are known as strict liability offenses.
Others may require proof the act was committed with such mental elements such as "knowingly" or "willfulness" or "recklessness".
[1] Although most legal systems recognize the importance of the guilty mind or mens rea, exactly what is meant from this concept varies.
The American Law Institute's Model Penal Code has reduced mental states to four.
Together or in combination, these four attributes seem basically effective in dealing with most of the common mens rea issues.
For example, threats, perjury, conspiracy, and solicitation are offenses in which words can constitute the element of actus reus.
A causal relationship between conduct and result is demonstrated if the act would not have happened without the direct participation of the offender.