Texas Penal Code Section 8.03 deals with the concept of mistake of law as a defense in criminal prosecutions. Here's a summary of its key points:

Ignorance of the law is not a valid defense: It states that it is not a defense to prosecution if the person was unaware of the law's provisions after the law has taken effect.

Affirmative defense based on reasonable belief: An individual can use an affirmative defense if they reasonably believed that their actions did not constitute a crime and acted in reliance on certain sources:

An official statement of the law in a written order or permission from an administrative agency responsible for interpreting the relevant law.
A written interpretation of the law in an opinion from a court of record or a public official authorized to interpret the law.
Lesser included offense possibility: Even if a person's mistake of law is a valid defense to the primary offense charged, they may still be convicted of a lesser included offense that they would have been guilty of if the law had been as they believed.

This section outlines the circumstances under which a mistake of law can be considered a valid defense and the specific criteria for establishing this defense in Texas criminal cases.