Sec. 21. (a) No compensation is allowed for any condition of physical or mental ill-being, disability, disablement, or death for which compensation is recoverable on account of accidental injury under chapters 2 through 6 of this article.
(b) No compensation is allowed for any disease or death knowingly self-inflicted by the employee, or due to his intoxication, his commission of an offense, his knowing failure to use a safety appliance, his knowing failure to obey a reasonable written or printed rule of the employer which has been posted in a conspicuous position in the place of work, or his knowing failure to perform any statutory duty. The burden of proof is on the defendant.