“Where ignorance is bliss,” wrote Thomas Gray back in
the 18^th Century, “’tis folly to be wise.”

But ignorance of
the law may not be bliss.  The
crucial difference to consider is whether the ignorance or not knowing is of
the law or of a particular material fact. 

In English common law, the principle was ignorantia juris non excusat, which
meant that lack of knowledge about a legal requirement or prohibition is never
an excuse to a criminal charge.  This
made a lot more sense back in the time when the list of crimes essentially
represented current morality.  Examples
would be easily recognized sins such as murder or theft.  These are called malum in se, which means a crime or act which is inherently
immoral. 

These days we have many more offenses that result
from administrative or social regulation.  These are malum prohibitum, acts
which are crimes merely because they are prohibited by law but not necessarily
immoral.  Examples might include
jaywalking or speeding. 

Ignorance of the law is no excuse, and that is even
if that old maxim is all one actually knows about the law.  So whether the law is malum in se or malum
prohibitum, one cannot hide behind one’s blissful ignorance and expect to
be legally excused.

But ignorance of fact may be different.  At common law, ignorantia facti excusat meant that
ignorance of a material fact is an excuse.  Here’s an example:  you are an
innocent pedestrian on the street who happens to possess a concealed firearm
permit.  You hear running approaching
from behind you and, turning, see a wild-eyed man drawing closer to you and
pointing a gun at you.  You draw your
pistol to defend yourself and, when he keeps coming, at about ten feet you pull
the trigger and kill him.  Self-defense,
right?  Well, in actuality, the guy
wielding the gun was going after someone other than you who was walking
nearby.  Your mistake of fact would be
your potential bridge to your self-defense argument. 

It is because ignorance of a fact may be a
sufficient excuse that we have the relatively recent phenomenon called plausible deniability.  This is a means of protecting the guy at the
top from prosecution if he can reasonably claim that he had no idea (of a
material fact).   And if this sounds like
a movie you’ve seen recently, I’m sure you have.