Texas Code of Criminal Procedure - CRIM P Art. 27.17. Plea of not guilty construed

The plea of not guilty shall be construed to be a denial of every material allegation in the indictment or information. Under this plea, evidence to establish the insanity of defendant, and every fact whatever tending to acquit him of the accusation may be introduced, except such facts as are proper for a special plea under Article 27.05.