There are countless laws. Literally.
Maybe not in the mathematical sense — it is technically possible to count the laws in existence, but based on a colloquial and general use of the term “countless” it is not really feasible for someone to count every law. Just this year (2011), forty thousand laws were passed and are set to go into effect. On the other hand, we rarely hear news about laws being repealed.
At this rate, if it is a monumental task to even count the laws, certainly, one can never know all the laws. And then — even if one knows generally of many laws, it is further impossible to understand the laws with requisite detail so as to ensure compliance.
Yet, people are told over and over by police, prosecutors, and the justice system that “ignorance of the law is no excuse.”
To all the police, prosecutors, asshole judges, and other grovelling subjects who insist on the “rule of law” — actually, yes, ignorance is an excuse. Perhaps one would be viewed as disingenuous when claiming he or she did not know murder, theft, or rape was against the law. However, when forty thousand laws are passed each year, each with possibly hundreds of pages of details, it is a perfectly legitimate excuse to claim one is ignorant of said laws.