http://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat
Ignorantia juris non excusat or Ignorantia legis neminem excusat (Latin for "ignorance of the law does not excuse" or "ignorance of the law excuses no one") is a public policy holding that a person who is unaware of a law may not escape liability for violating that law merely because he or she was unaware of its content; that is, persons have presumed knowledge of the law.
The rationale behind the doctrine is that if ignorance were an excuse, persons charged with criminal offenses or the subject of civil lawsuits would merely claim they were unaware of the law in question to avoid liability, whether criminal or civil. Thus, the law imputes knowledge of all laws to all persons within the jurisdiction no matter how transiently. Even though it would be impossible, even for someone with substantial legal training, to be aware of every law in operation in every aspect of a state's activities, this is the price paid to ensure that willful blindness cannot become the basis of exculpation. Thus, it is well settled that persons engaged in any undertakings outside what is common for a normal person, such as running a nuclear power plant, will make themselves aware of the laws necessary to engage in that undertaking. If they do not, they cannot complain if they incur liability.
http://en.wikipedia.org/wiki/Mens_Rea
The mens rea is the Latin term for "guilty mind" used in the criminal law. The standard common law test of criminal liability is usually expressed in the Latin phrase, actus non facit reum nisi mens sit rea, which means that "the act does not make a person guilty unless the mind is also guilty". Thus, in jurisdictions with due process, there must be an actus reus accompanied by some level of mens rea to constitute the crime with which the defendant is charged (see the technical requirement of concurrence). The exception is strict liability crimes (in the civil law, it is not usually necessary to prove a subjective mental element to establish liability, say for breach of contract or a tort, although if intentionally committed, this may increase the measure of damages payable to compensate the plaintiff).
Quite simply, therefore, mens rea refers to the mental element of the offence that accompanies the actus reus. In some jurisdictions, the terms mens rea and actus reus have been superseded by alternative terminology. In Australia, for example, the elements of all Federal offences are now designated as "Fault Elements" (mens rea) and "Physical Elements" (actus reus). This terminology was adopted in order to replace the obscurity of the Latin terms with simple and accurate phrasing.1
There are four general classes of mens rea (the words used may vary from one state to another and from one definition to another) but the substance is:
Intention; (purpose)
Knowledge;
Recklessness sometimes termed willful blindness which may have a different interpretation in the United States; or
Negligence.
