If the scientists can say this when they talk about small things like photon and germs, we, as lawyers, can say this when we talk about the law. This is one of the most misunderstood piece of law which is in place everywhere you go, whether you are a resident of the country or someone who just came into the country. This law is where not knowing the law of the land where you have just comes into, does not make you unaccountable for any crime which that country has labelled as a crime. In short, if you did something which may be legal in your country, but you did not know that it is a crime where you have just landed.
The maxim which lawyers love to quote is : Ignorantia Legis Non Excusat which is a latin word that basically means, Ignorant of the Law is No Excuse. Layman term, not knowing the law does not excuse you from being charge with a crime (or found guilty of it).
You can pretend that you don't know the law of the land which you have just entered. How many times have you read people who are conned into bringing drugs into other countries and get caught for it. They are called Drug Mules and they are usually gullible teenagers who were tricked by casanovas or beautiful girls. When any of them are caught, although some do know that they are being used for bad intention, they are still charged in court for bringing into the country illegal contraband. Even if they are found innocent and does not know that they are doing something illegal, it does not release them from being liable.
In Malaysia, Singapore and Thailand, anyone bringing in drugs can be sentenced to death and cases like this have been heard so many times. Maybe as human, you can say that it is a harsh penalty but it is a fact that drugs is a menace to most Asian countries and have been viewed as something governments want to curb. So, again, not knowing does not release you from liability of a criminal charge.