General causes of challenges are:
1. A conviction for felony.
2. A want of any of the qualifications prescribed by law, to render a person a competent juror, including a want of knowledge of the English language as used in the courts.
3. Unsoundness of mind, or such defect in the faculties of the mind or organs of the body as renders him incapable of performing the duties of a juror.
R.L.1910, § 5857.