1. A person is guilty of arson in the third degree when he intentionally damages a building or motor vehicle by starting a fire or causing an explosion.

2. In any prosecution under this section, it is an affirmative defense that (a) no person other than the defendant had a possessory or proprietary interest in the building or motor vehicle, or if other persons had such interests, all of them consented to the defendant's conduct, and (b) the defendant's sole intent was to destroy or damage the building or motor vehicle for a lawful and proper purpose, and (c) the defendant had no reasonable ground to believe that his conduct might endanger the life or safety of another person or damage another building or motor vehicle.

Arson in the third degree is a class C felony.