Ignorance of the law is a valid defense in criminal cases. It stems from the principle of mens rea (a guilty mind. That a reasonable person should have known [act] is criminal. For example, if you murdered someone you couldn't say "oh, I didn't know that was illegal." The standard of mens rea would be met. But, for a violation of a technical detail, such as possessing stolen property, it is possible to have good intentions and simply be ignorant of the law.).
The phrase "ignorance of the law is no excuse" arises from civil cases. Just because you're ignorant of a trip hazard on your front walkway will never absolve you of your liability for injuries arising from that hazard.
Aristotle’s theory of voluntary and involuntary actions performed by man by which he is responsible is important because it seems,
IMO, the platform from which the American legal system is based – ignorance of the law is no defense. Things done from constraint or ignorance are considered involuntary actions, while things free from force or pressure are coined voluntary actions. Whether involuntary or voluntary, one is responsible for his actions even if ignorance of the elements warrantied may have stopped the person from engaging in such action – no defense. The deed is done, the action was performed- involuntary or voluntary, and the actor is at fault for its result. In essence, a person is presumed to know or should have known the law and is responsible for the actions whether culpable or not.
"But for..."
Causation... Causal relationship between Conduct & Result.
To establish causation it is a 2 stage inquiry. #1 Factual causation & #2 Legal causation.
Did the Person act in the other persons loss? This must be establish First before #2 can be reviewed.
If #1 is deemed then a look into #2 - Does the law apply in some fashion?
Mens Rea is simply an element of causation which there is its counter-part - Actus Reus.
Actus Reus (An Action) + Mens Rea (State of Mind) = Sine qua non?
In most cases these elements need not matter for Actus Prohibitum dictates many matters not "But for..." Matters!
"Everything done by reason of ignorance is involuntary. The man who has acted in ignorance has not acted voluntarily since he did not know what he was doing. Not every wicked man is ignorant of what he ought to do and what he ought to abstain from; by such errors men become unjust and bad."
- Aristotle