' No one is "presumed" to know the law; the reality is that knowledge of the particular statute prohibiting certain conduct is not usually an element of the crime created by that statute, and therefore knowledge of the law is not required for punishment.
Examples of mens rea in statutes
Model Penal Code: A person commits murder if he (1) purposely or knowingly (2) causes the death of a human being.
Common Law: (a) It shall be unlawful for a person to cause the death of a human being with malice aforethought. (b) A violation of this section is murder in the second degree.
See generally: Criminal law