Mistake of Fact vs. Factual Impossibility

Could you clarify the difference between mistake of fact defense and factual impossibility and how they each apply (or don’t apply) as a defense to specific intent and general intent crimes?

Response:

It's very important to keep the differences here in mind, for the exact reason stated in the question. Specific intent crimes, and general intent crimes, are affected by mistake of fact in a way that they are not affected by factual impossibility.

Mistake of fact is relevant to a defendant's criminal liability only if it shows that the defendant lacked the state of mind required for the crime. So, for specific intent crimes, mistake of fact could negate the intent element, and that could absolve the defendant from liability for that specific crime. For example, assume that a person who buys stolen goods honestly and reasonably believes that the goods actually belonged to the seller. This would negate the criminal intent necessary to be convicted of receiving stolen goods, and the buyer would not be held criminally liable. Further, on the MBE, for specific intent crimes, the mistake of fact need not be reasonable; any mistake, reasonable or unreasonable will suffice. For general intent crimes (for example, false imprisonment), the mistake must have been reasonable.

On the other hand, factual impossibility will show up when defendant has attempted to commit a crime, but has failed to complete the crime because of his mistaken belief about the facts. (For example, a defendant who points a gun at the victim which he mistakenly believes to be loaded, and pulls the trigger, intending to kill the victim. In this case, defendant can be guilty of attempted murder, because, unlike with mistake of fact above, defendant here did have the required intent (ie, mens rea) to commit the crime.