Mistake of Fact

The common law has a doctrine called "mistake of fact." It is conceptualized as a defense, even though in reality the prosecutor has to prove the absence of the defense beyond a reasonable doubt. 

The mistake of law doctrine is confusing. We won't spend much time on it. It basically addresses badly what the MPC addresses well. Indeed, this confusing doctrine was one of the problems the MPC sought to solve.

The mistake of fact doctrine considers whether a person is guilty if they were unaware of some fact, like being within 500 feet of a school. Under the MPC, we would determine the mens rea for that element, 500 feet, and then see if the prosecutor can prove the defendant met that mens rea. The MPC supplied rules to decide the mens rea for that element. 

Under the "common law" mistake of fact doctrine, we proceed differently. We first determine if the crime is a specific intent or general intent crime. That is usually, but not always, straightforward. Then the doctrine tells us a different mistake of fact rule applies for each of those two categories of crime. Each of those two rules is analogous to supplying a mens rea for that element, but it's best to just use the language of the common law mistake of fact doctrine. This will become more clear when you read the cases below.