Ask Lemmy

It's absolutely essential. Otherwise every single defendant would claim "I'm sorry, your honor. I didn't know that murder/rape/stealing is illegal". It's almost impossible to prove or disprove general knowledge so we must assume - at least from a legal perspective - that everybody knows the law if we want to apply that law at all. Of course that doesn't mean that any random person on the street needs to be familiar with every single law that applies to forestry but that they are required to read up on those laws before they pick up a chainsaw and head into the forest. There may be a few obscure laws that could apply to you without your knowledge but those are mostly so low stakes that we can give people a warning for the first offense and then reasonably assume they will know and follow that law in the future.

Now, that all applies to ignorance of the law. On the other hand, there is ignorance of your own actions which indeed can get you out of a conviction because it indicates a lack of intent. A simple example: if you visit someone and on the way out, you grab their jacket because of your own because they look similar, it's very likely that you haven't done anything illegal, it was just a mistake. Same if you're not aware that something you're doing might endanger others. Those might still get you in trouble for negligence but one could reasonably construct a case where you do something that looks perfectly safe to you but out of pure coincidence ends up killing someone. In those cases, you're clearly not guilty of a crime.