Tex. Penal Code § 8.02: MISTAKE OF FACT.

(a) It is a defense to prosecution that the actor through mistake formed a reasonable belief about a matter of fact if his mistaken belief negated the kind of culpability required for commission of the offense.

(b) Although an actor's mistake of fact may constitute a defense to the offense charged, he may nevertheless be convicted of any lesser included offense of which he would be guilty if the fact were as he believed.

Acts 1973, 63rd Leg., p. 883, ch. 399, Sec. 1, eff. Jan. 1, 1974. Amended by Acts 1993, 73rd Leg., ch. 900, Sec. 1.01, eff. Sept. 1, 1994.