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.
In addition, there were, particularly in the days before satellite communication and cellular phones, persons who could genuinely be ignorant of the law due to distance or isolation.
For example, in a case in British Columbia, four hunters were acquitted of game offenses where the law was changed during the period they were in the wilderness hunting.
The doctrine, "Ignorance of the law is no excuse", first shows up in the Bible in Leviticus 5:17: "If a person sins and does what is forbidden in any of the LORD's commands, even though he does not know it, then realizes his guilt, he will bear his iniquity."
An alternate explanation of the origin of the maxim, though not particularly relevant to the modern context, can be found in the philosophy of the Greeks and Romans.
We find that Cicero wrote the following in De re publica (On the Republic): There is a true law, right reason, agreeable to nature, known to all men, constant and eternal, which calls to duty by its precepts, deters from evil by its prohibition.
On the penal side, the quality of the knowledge of the law can affect the evaluation of the animus nocendi or the mens rea, in that certain subjective conditions can weaken personal responsibility.
It was then argued that both the presumed knowledge and the heavily increasing corpus of national legislation were working in favour of lawyers rather than citizens.
This principle is also stated in statutes: In some jurisdictions, there are exceptions to the general rule that ignorance of the law is not a valid defense.
For example, under U.S. Federal criminal tax law, the element of willfulness required by the provisions of the Internal Revenue Code has been ruled by the courts to correspond to a "voluntary, intentional violation of a known legal duty" under which an "actual good faith belief based on a misunderstanding caused by the complexity of the tax law" is a valid legal defense.
It was subsequently ruled in United States v. Freed (1971) that this exception does not apply when a reasonable person would expect their actions to be regulated, such as when possessing narcotics or dangerous weapons.