BS. How many laws are there on the books? NOBODY, not a single lawyer or judge, has ever read them all, much less understood the case law associated therewith.
The maxim is little more than authoritarian posturing before depriving its unwitting victims.
Oh really? Please do cite the successful defenses of a criminal violation by someone who was unaware of the statute.
For some, it's a real complicated issue.
Does it have pedals?
Does it have a headlight?
Does it have a tail/brake light?
Does it have turn signals
Does it have a license plate?
Is it registered?
Is it insured?
If the answer to the above questions are "No," and stay with me here, then it can't legally be operated on the street.
It doesn't take a Clarence Darrow to determine whether a motor vehicle is street-legal or not.